"Waiting for Guffman": what are they doing "wrong"?

It’s said that the show-within-a-show in “Waiting for Guffman” is even funnier if you’ve worked in theater, because you can recognize the various theatrical “don’ts” that the production is blithely ignoring. For instance, at the start of the play, all four actors enter and bow to the audience, then two of them immediately exit - something you’d never see in a play with competent direction (or so I’ve heard).

What are the other examples of this in “Guffman”? Not obvious screw-ups like when Eugene Levy’s character fluffs a line, but subtler cases of the cast and crew (of the play, not the movie) clearly not knowing what they’re doing?

Playing to the man in the critic’s reserved seat?

“Dybbuk, Schmybbuk, I said more ham!”