It happens, right angle, right distance and right time and its lights out.
Look at Dos Santos vs. Velasquez in their first fight. Junior caught him just right with a wild looping punch and TKO'd him. Next 2 fights, Velasquez tore him apart.
Look at Kimbo Slice vs. Josh Thompson (i think), caught him just right on the chin with a weak looking punch and KO'd Kimbo.
Those are just two quick examples that come to mind in the beginning of a HW fight.