Question:
I accidentally used a clean fleishig (meat) knife to slice a tomato for my cheese sandwich and only realized afterward. I put the tomato into my cheese sandwich, and the sandwich went into the toaster/sandwich maker. Am I allowed to eat it?
Answer:
As long as your knife was presumed clean — meaning it went through a normal scrub-washing with soap and water — it is permitted for you to eat your sandwich.
Explanation:
Although according to many Poskim it is initially forbidden to cut a pareve food with even a clean knife of the opposite type (for example, cutting a tomato with a meat knife in order to use it in a dairy meal), this restriction is lechatchila (initially). After the fact (bediavad), if it occurred, the food remains kosher. There is no concern that actual meat substance transferred from the clean meat knife into the tomato. Therefore, if the knife can be assumed to have been washed and is clean, everything remains permitted. Although some poskim are stringent when the knife is ben yomo (used within the last 24 hours), this strict view applies only initially, and most poskim do not rule this way after the fact.
Sources:
See regarding initial prohibition to cut even a pareve food with a knife of the opposite type: Assur: Shach 89:22; Nekudas Hakesef ibid “Even to cut bread to eat with cheese requires Neitza”; Toras Chatas 76:6; P”M 89 S.D. 22; Kneses Hagedola 89:16; Toras Yekusiel; Beis Yitzchak 89:7 Mutar: Taz 89:6 “However, regarding cutting bread, only a cleansing of the knife is required” and 89:7 “According to the custom, one may not use the knife to cut cheese even if Neitza is performed, however in my opinion, he may use it to cut bread if it is cleaned”; [no differentiation is made regarding Ben Yomo or not]; Implication of Beis Yosef 89 in name of Orchos Chaim in name of Rabbeinu Shimshon; Halacha Pesuka 89; Chochmas Adam 40:14; Aruch Hashulchan 89:16
See regarding Bedieved that if the knife was clean the food remains Kosher: See Taz 89:6 and 89:7; Shach 96:6 and 21-23 that the rinsing is a mere Chumra; Peri Megadim 96 A”A 21; Halacha Pesuka 89; Chochmas Adam 40:14; Aruch Hashulchan 89:16; See Hakashrus 10:87 that today we consider a knife to be clean when washed with soap, and thus if the knife is clean, a Pareve food which was cut with that knife may be eaten with the opposite food even if the knife did not have Neitza done. One is not even required to wash the Pareve food.
