Sometimes a few small mistakes can ruin a class. Sometimes big ones don't. It depends on the class and the mistakes. For the theatre tour class, I've made a number of mostly careless mistakes, but some have had a large impact. For those unfamiliar with the course, the concept is a sort of page-to-stage thing, where we read several plays and then go see them in performance. Once I took student to London; this semester we're seeing regional theatre.
Mistake 1: Inconsequential: In the syllabus I noted merely that a show opened on a particular date, because I hadn't yet secured tickets. Even after I announced in class the actual date, one student told me that she had already made plans around the opening. not a mistake so much as a miscommunication.
Mistake 2: The syllabus says that our second Spring break trip is the final Saturday and sunday of break, when in fact they are the final Friday and Saturday. Not only does this conflict with at least one studennt's plans, it is the same student as in #1. This one is all my fault.
Mistake 3: Next weekend, we're seeing 1 Henry VI. I was so excited a few weeks ago to teach Hal and Hotspur again. VI? IV? crap.
I've had lots of these students before, some several times, so they're very forgiving of my quirks, but these, well, these are doozies...