This Video discusses three inappropriate things to do while performing prayers.

The first thing is to look around during prayers. It mentions that Prophet Muhammad (peace be upon him) said that looking around is a portion that the devil snatches away during a man's prayer.
The second thing is to perform rakus (bowing) and sajdas (prostrations) swiftly. It states that Prophet Muhammad said that the worst thief is someone who steals his own prayer.