Never demonstrate how to perform a split jerk when wearing socks on a smooth floor. Fortunately I am flexible, as I split far more deeply than I had planned.
Never assume that you can tell how much weight is on a bar just by looking at it. When your "friends" change the weight in between sets during a snatch workout, and forget to put it back after taking a light warm-up set, the bar can achieve a velocity just short of incredible. It is amazing how throwing a lightly loaded barbell through the wall in back of you will irritate people.
Always blow your nose before attempting a deadlift pr if you are having sinus trouble. Sneezing at the top with your hands full means that you are going to spray somebody. And your wife will never let you forget it. And make you pay to get her hair done.