Someone I know almost couldn't become a lawyer because he had a delinquent record, because he "pranked" another kid in middle school by putting purell in his drink and the kid got sick. Don't fuck with people's food as a prank.
Another person I know went into anaphylactic shock from eating shellfish that someone snuck into her food. Up until that point she didn't know she was allergic to shellfish, because she keeps kosher and shellfish isn't kosher, but the person making her food didn't take that seriously. Don't fuck with people's food because you are apathetic towards or resentful of their dietary preferences.
Someone else I know went to the hospital because she's celiac and ate something erroneously labeled gluten-free. Another person was fired from his job and beat up by other teens when he spit in a pitcher of orange juice while working as a camp waiter. Still another person I know transferred universities after being tricked into eating something that was not kosher. The list goes on and on.
Don't fuck with people's food because you think it's funny, or for petty revenge, or because you think they won't notice anyway. Don't fuck with people's food for any reason, because dietary needs – and preferences – are important and have real-world consequences.