Just been reading back through this thread and thought of more!
On beam:
Squeeeeeze everything!...OK, Not your eyes!!
"Are you drunk?" Or "Did you have wine in your drinks bottle today?" (If they are extremely wobbly!)
In general:
 
"REEEAAALLLLYYY???"  This has become a bit contagious in our gym - one coach started saying it when a gymnast did a skill and it went wrong, now everyone says it, not particularly funny, but just something we all say! (Kind of similar to 'Seriously?!' in Grey's Anatomy)
I also ask "Is that really how we do that?" 
When we are packing away: 
"insert name stop supervising, that's my job!"
"That mat isn't going to move itself"
And of course I have many more positive ones too, but they aren't as fun!