The Alter Rebbe states that the Arizal said that on Pesach one should be stringent like all the stringencies, as one who is careful to avoid even a speck of Chametz is guaranteed not to sin throughout the year. This means that his nature will change to the point that it is so refined that he will not naturally do a sin inadvertently. However regarding sinning advertently, one always retains his freedom of choice.
Being Machmir not to offend others with your Chumros:
It once occurred by a Pesach meal on the table of the Rebbe Rayatz in New York that a certain uneducated guest dipped his Matzah in the Borscht [beet soup], performing a grave sin in the eyes of the Chassidic brotherhood that were present by the meal. As can be understood, a great tumult transpired surrounding the actions of this guest, and the other Chassidim present gave the guest a piece of their mind. The Rebbe Rayatz inquired as to the reason behind the commotion and was told of the grave actions performed by the guest on his very own Pesach table. The Rebbe Rayatz nonchalantly replied “It is better that the Matzah become red [with the Borscht soup] than you cause the face of a Jew to redden in shame.”
A. Matzah Shruyah/Gebrochts:
What is it? Matzah that has come to contact with water either by cooking with water or dipping in water.
The Law: The Chassidic custom is not to eat any Matzah dipped in water due to a suspicion that part of the flour may not have been kneaded into the dough and thus now when it will come into contact with the water it will become Chametz. “Now although that this is not a complete and clear prohibition according to the letter of the law, nevertheless one who is stringent is blessed, and is not considered to be a wondrous person which does things without reason, as there is a great reason involved in order to avoid a suspicion of eating flour which was not kneaded into the dough and then came into contact with water, which is a Biblical prohibition according to many Rishonim.“…. “However one should not to protest against those that are lenient as they have upon whom to rely, mainly the Rambam and Rashi, although according to what the Arizal writes that one should be stringent on all the stringencies of Pesach, then it’s obvious that one should be stringent.”
On the last day of Pesach: On the last day of Pesach in the Diaspora one who is lenient to eat matzah with water for the purpose of Yom Tov joy, is not losing out on keeping of the above mentioned stringencies of the Arizal. Our Holy Rabbeim were even scrupulous to dip their matzahs in liquids, and with each different food, in fish, meat, and especially soup. Even those foods which throughout the year are not normally eaten with bread, they would eat with their Matzah.
The Chabad custom: We are so stringent in the above that we make sure to place our matzahs in bags when we eat them, lest a crumb of it enter the food. Similarly we check our cups and plates before we eat from them to make sure
Dipping Matzah in Fruit Juices: “Regarding dipping matzah in fruit juice it’s obvious that one need/should not be stringent against doing it throughout the entire Pesach.” The Rebbe Rashab would eat his Matzahs with wine [and milk]. If fruit juice is mixed with water then it is even more of a problem then plain water regarding its ability to ferment, being that plain water takes at least 18 minutes to ferment, while when mixed with fruit juice it can ferment immediately. Thus care must especially be taken that when eating matzah with fruits etc, that there is no water around, including perspiration, on the fruit. Practically one may eat Matzah with avocado, tomato, oil. But one must beware that cheese and butter may not be placed on the Matzah as they contain water.
Eating on vessels used with Gebrochts: The custom is to not use any vessels on Pesach which had wet matzah fall on it that Pesach. However from a previous Pesach, it is not a problem to use even though no koshering was done.
May children eat Gebrochts on Pesach? In a letter from the earlier years the Rebbe writes that one may be lenient to with regards to children in feeding them Gebrochts. However in a Sicha of 1988 the Rebbe states that children should not eat Gebrochts on Pesach, being that Matzah is Emunah, and with faith one cannot be lenient. [However babies which will not be able to eat anything else, may eat Gebrochts, and it is better to eat it then to eat kitniyos.]
May one take a bite off a large piece of Matzah or is one to only enter small pieces into his mouth? Some are accustomed to only enter bite pieces of Matzah into their mouth at a time, in order to prevent saliva [which is equivalent to water] from coming into contact with the bitten area. Rav Groner told me he never heard of such a custom.
B. Processed foods: 
It is a renowned Chabad custom to avoid eating all processed foods on Pesach.
Matzah and wine: This is with exception to Matzah and wine, which due to inability to self produce, is widely purchased from a store or company. Nevertheless many families are stringent to produce their own wines and even bake their own Matzas.
Oil: The Rebbe writes that “Anash use Natala margarine on Pesach”. This refers to a congealed vegetable fat that was processed by a company under a local Mehadrin Hashgacha. Thus one may purchase any Kosher for Pesach oil that contains a most reliable Hashgacha. Nevertheless there are those which are stringent to only use melted chicken fat [Shmaltz] as their oil base product for Pesach food and cooking.
Sugar: See above!
From what time are those that are accustomed to not eat processed foods to begin their stringency? From the night of Pesach or from 5th hour of Erev Pesach?
One who is stringent to avoid eating processed foods is to avoid eating them starting from the 5th hour of the day.However many are lenient to eat processed foods up until the night of Pesach. 
 Shut of Admur 6
 Hisvadyus 5758, page 171
 Sefer Haminhagim
 Sefer Hasichos 5703 p. ??
 Sefer Haminhagim
 Otzer Minhagei Chabad p. 48.However tzaruch Iyun as to the source of this custom, as no mention of the source is made there
 Shaar Halacha Uminhag
 The source of the custom:
I have not found this custom recorded in any compilation of laws dealing with the Chabad Pesach customs. It is not mentioned in any of the following Sefarim: Sefer Haminhagim; Otzar Minhagei Chabad; Shevach Hamoadim. Vetzaruch Iyun. Seemingly the reason behind this custom is because we are accustomed, based on the Alter Rebbe, not to eat at other people’s houses on Pesach due to that we are not aware of their level of stringency. Thus we avoid eating processed foods being that it is equivalent to foods made in other people’s homes. Alternatively the reason behind this custom is because we suspect that perhaps in the production line, which involves hundreds of people of various backgrounds, chameitz fell into the food and has not disintegrated. [This can easily occur if a worker had a sandwich and did not wash his hands properly afterwards. Having worked as a Mashgiach in a Kosher for Pesach I can attest that it is almost impossible to ascertain that all the workers wash their hands from all Chameitz leftovers prior to beginning work on the line.] According to this reason one is to avoid processed foods beginning from the time of prohibition to eat Chameitz from the 5th hour of Erev Pesach, as Chameitz Beiyn which is found in a food is at times not nullified even before the night of Pesach. [See 442 Kuntrus Achron 15; 466/9-11; Shut Rabbeinu 18; See Piskeiy Admur Yoreh Deah chapter 100; Shach 109/8; “A Semicha Aid for learning the Laws of Taaruvos” Chapter 104/4]Alternatively, perhaps we even suspect for disintegrated Chameitz as we suspect for the opinion in 447/22 that holds that we say Chozer Veniur by all foods that contain Chameitz of any amount, even taste, even if it was nullified before Pesach. According to this reason one is only to be stringent in avoiding to eat processed foods beginning form the night of Pesach.
 Igeres Hakodesh Vol. 21 p. 96. This was a written telegram to Anash of some part of America or Canada in 1949.
 Response of Rabbi Yehuda Leib Groner [secretary of the Rebbe] to a query of the author; Custom of many of Anash from vintage Chabad homes as heard by the author
The reason: The reason for not eating processed foods is due to a suspicion of Taaruvos Chameitz. Now before the night arrives, although Chameitz can be nullified, this only applies if it is a mixture of Yaveish Beyaveish or Lach Belach. However by a mixture of Yaveish Belach it is not nullified even before Pesach. [442 Kuntrus Achron 15; 466/9-11; Shut Rabbeinu 18; See Piskeiy Admur Yoreh Deah chapter 100; “A Semicha Aid for learning the Laws of Taaruvos” Chapter 104/4] Thus if there is a crumb of Chameitz that fell into the processed food and is still intact it is not nullified and one transgresses eating Chameitz on Pesach. Now seemingly
 Custom of many families of Anash.
The reason: Seemingly the reason behind the leniency on Erev Pesach until the night, despite that which is explained above, is because on the night of Pesach begin the severe stringencies of Issur Mashehu, and Issur Kareis for eating Chameitz. Thus although from a Halachic perspective there can be suspicion of Chameitz in processed foods even before the night of Pesach in a way that it is not nullified, nevertheless one only begins to suspect for this starting from the night. Vetzaruch Iyun. Alternatively, perhaps it is because we suspect for the opinion in 447/22 that holds that starting from the night of Pesach we say Chozer Veniur by all foods that contain Chameitz of any amount, even taste, even if it was nullified before Pesach.