Good question, actually. The joys of directing. One time our production team were in the middle of a zombie film (Couple years back, totally a great learning experience, but terribly under-experienced and unprepared), and we were putting make-up on one of our actresses, and she had a full-on sezure right before our eyes. Got scary. Guess that isn't so bad, now that I think of it. Was crazy at the time.
One of our actors had a mental breakdown on time, and started losing his entire mental capacities outside of our filming location, trying to bust inside, banging on the door like a madman, trying to rip the cast to shreds. Luckily that didn't happen, but it was pretty crazy. Stupid, most-like-it.
During a scene from our film "Renegade Rat", one of the 'lead' supporting actors (definitely an important role) dropped out the day of filming, so we had to find a quick replacement. The one fellow, who I'd originally offered the role, said he still didn't want to do it, but he'd show up and be an extra. When he came to the set, he was drunk like a mad hatter, and I gave him all the lines from the supporting character, and we went over it for quite a few hours. Finally, a couple days later, I told him what I'd done. He had a change of heart, and wanted to role, so we continued doing scenes with him in the role. Couldn't believe how that one turned out. Got lucky.
Directing is always so entertaining (along with so many other things), and there are lists of things I could go on about, but I won't be self-indulgent and pretend my situations have been any worse than anyone else's. I've enjoyed reading through this thread, though.