My younger bro came up with the best ad-lib to the scripted yell-ats one night ... the scenes when they're in the pool, at the end of all the "Waiter, waiter, there's a fly in my soup" stuff. After the audience yells, "Waiter, waiter, there's a dead fly in my soup" (pause) "What do you want, a refund?" Bro takes a two-beat and yells into the silence, "No, salt and pepper."
The theater erupted, they laughed so hard at that! We had people coming up to us after the movie to slap him on the back, "Great line, can I use it next week?" LOL!!
It's at that point we would yell "NOT MEATLOAF AGAIN!!!"