Justin Timberlake -- The Cracked Out Bachelor Party Weekend

Justin Timberlake refused to make a complete ass out of himself during his bachelor party weekend in Cabo San Lucas this weekend ... only going "partial crack" while his pals flashed some full moons at the paparazzi.

Prior to flashing their lily-white booties, JT and the gang started the bachelor festivities in Las Vegas on Thursday before continuing the party train south of the border.