When you're on the "at this performance" insert in the program

When your costume fitting, spacing rehearsal, and first time with the almost-full cast for a ballet all take place the day before the performance (in a studio, not on stage), and you never run the show with a truly full cast, or on stage, or in your costume, until 8pm the next evening in front of an audience, and you learn at said spacing/run-thru the night before the show that you're being added to a piece you weren't supposed to be in, in which you have to wear a mask that results in you having no peripheral vision.... That's when you really really really appreciate the fact that swings and understudies sometimes have the most difficult jobs in theatre.  Oh, and my first quick-change (never rehearsed) I was kind of nervous about making in time.  I finished just in time, was like "Yay, I made it!", went straight on stage, only slightly tripped on my dress one time when I was on my knees (not bad for having never rehearsed in it), was thinking, "Gee, this is cut awfully low in front. I don't remember it being that low at my fitting yesterday.  Good thing I have a black leotard as a base," and then came offstage, where one of the other dancers took one look at me and said, "Your dress is on backwards."   ....I once stage managed a show where an actor put his pants on backwards in a quick-change during a performance and came on stage like that without realizing.  I laughed and laughed and asked how he did that without noticing..... Dude....I totally get it.