Not to touch the washing water:
One must be [very] careful not to touch the water in the vessel before washing as one who does so contaminates the water with his hands [and is considered as if it was already used and is hence invalid for washing]. Thus one must be very careful while washing with the vessel that his fingers do not touch the water in the vessel. [See Q&A regarding if one touched the water prior to going to sleep.]
If before going to sleep one touched the water set aside for washing may it still be used to wash with in the morning?
If prior to going to sleep one accidently touched the water in the basin prepared for the morning washing, the water nevertheless remains valid. Furthermore even if one intentionally scooped some water out of the basin prior to going to sleep the water nevertheless remains valid. However if one cleaned his hands in the water that is in the basin, then even if this was done prior to going to sleep the water may not be used, as is the law regarding washing hands in the morning.
What is the law if after the first pouring some of the water on one’s hand fell into the remaining water in the cup?
Seemingly the remaining water becomes impure and may not be used.
 Siddur; Basra 4/2, Kama 4/12
 Basra 4/2
 Siddur and Basra 4/2, However Kama [4/6] does not warn against doing so, rather just simply states in 4/12 that if one does so the water is contaminated.
 Touching after washing: However if one already washed, touching other water accidently does not contaminate it. That which is explained in Beis Yosef; Shulchan Gavoa 1/8; Kaf Hachaim 4/25 that even after washing one should not touch it, is referring specifically to beer, as touching it can ruin it. It has no relation to the current Halacha. [Mamar Mordechai 4/5]
 Kama ibid
 Basra ibid
What is the law if one accidently touched the water: From Admur ibid it is implied that even if one accidently touched the washing water it becomes impure. However the Ashel Avraham Butchach 4 rules that only when one touches the water intentionally does it become impure as opposed to one who touches it by mistake. [See Piskeiy Teshuvos 4 footnote 158]
 Based on 160/6 unlike Piskeiy Teshuvos 4/16 in name of Ashel Avraham Butchach 4
 As only if one touches the water after awakening with the impure spirit does entire water become impure and may not be used for washing as explained in B.
 Based on Halacha B see there!