Going over 21 (a total of 22 or more) and losing.
If you bust, you lose even if the dealer busts afterward. This order of play is the main source of the house edge. The dealer busts about 42% of the time with a 6 up and about 12% with an ace up.
At the table
A player with 10 and 6 (16) hits and draws a 9 for 25, busting on the spot. The bet is taken at once and is not returned even if the dealer busts later.