#1 bad phrase:
"I shouldn't say this, but..."
Once you've said that, you need to stop right there. Better to have them guess what awful shit you were going to say than to actually say it.
My former boss was big with this one. Next thing out of his mouth was ALWAYS brutally inappropriate, including, "... we really hit the client's G-spot with this one."
A little part of me never came back after that.