Thinking you are going to need to write a lot of notes for a meeting and setting up an entire page, only to write one thing on it the entire time...
I HATE this- it happens in Rehearsal, Production Meetings, and Class.
Also, hearing a soloist start a song a thirty-second of a beat late.The audience may not catch it, but you remember the entire run of a show.
Asking an ASM to finish some paperwork, only to find that they did it halfway, handed it to the other ASM, who then doodled on it and asked an actor to return it to you.
Blocking page-by-page, then discovering that you blocked two pages on one sheet.
An actor that misses their light- AFTER it is spiked for their convenience