I was sat in the upper circle at the Old Vic a while back when there was a particularly heavy rain storm & water started leaking through the ceiling & onto the audience a few row in front of me. It just stuck in my mind because I remember feeling really sorry for them!
Not sure if it's these kind of leaks that would weaken plaster & eventually break away.
A while back when we went to see Trelawny of the Wells at the Donmar & I had an empty seat between myself & the next woman along. I was enjoying the extra space for a chnage then just as the lights went down for the second half, two girls sprung down from the standing section at the back & squeezed themselves into the one vacant seat! As the lights had gone down & Act 2 had started I couldn't say "what are you doing?!" So spent the second half with one of their butt cheeks practically in my lap. No amount of shuffling or sighing seemed to indicate to her I was uncomfortable & at no point did she say "is it ok we sit here or do you mind being squashed for the rest of the play?"