"Are you sure?"
If I volunteer to help, and someone asks, "Are you sure?", I get
a bit annoyed because if I offered to help in the first place, it
means that I want to help. So, sometimes, when my colleagues
do that, I'll say, "Actually, I'm not sure! I'm never sure if
I really want to help." 
Needless to say, cheeky remarks like that can only be made with
colleagues that I'm close to. 
"You're alright!"
Over-used to death here. I hear it so much that I found
myself using that meaningless phrase a few times too!
