"I have 'the actors nightmare' several times a year: I'm being led to the stage. I don't know my lines, or the plot, and I'm trying to read a script, but it's too dark to see the lines..."
My nightmare is it's opening night, and I know I forgot something...what was it...oh, that's right! I forgot to GO TO REHEARSAL (for the prior few weeks, no less)! So I keep rummaging through the script, and can never find where my lines are.
Oddly enough, that actually happened to me. I was understudying for an actor who had to be out town for a few days during the run, and about a week before that was to happen, I got an emergency call he had a kidney stone, and I'd be going on that night. I suited up, set my script backstage, and got ready. It went ...okay. I missed closing a door during a blackout, and was beating myself up backstage when one of the actresses in the show hissed "Dan...don't you go on in a half page?"
I freaked out, and started panic-flipping through my script...not being able to find the page with my lines on it. The classic Actor's Nightmare.
It turns out she'd forgotten there was another page of dialogue before my entrance.
But I still have nightmares about the nightmare.