This question is targeted at people who assign problems: teachers, student assistants, tutors, etc.
This has happened to me a handful of times in my 12-year career as a professor: I hurriedly assigned some problem from the text thinking "this looks good." Then later realized I couldn't solve it. Few things are more embarrassing.
Here's a recent example: "Give a linear-time algorithm that determines if digraph $G$ has an odd-length cycle." I assigned this thinking it was trivial, only to later realize my approach wasn't going to work.
My question: what do you think is the "professional" thing to do:
- Obsess on the problem until you solve it, then say nothing to your students.
- Cancel the problem without explanation and move on with your life.
- Ask for help on cstheory.SE (and suffer the response, "is this a homework problem?")
Note: I'm looking for practical and level-headed suggestions that I perhaps haven't thought of. I realize my question has a strong subjective element since handling this situation involves one's own tastes to a large extent, so I understand if readers would prefer to see it not discussed.