Pick the Medic, not know how to heal, and just melee everything.
Pick the Pyro, use magma instead of fire. MELT EVERYTHING!
Pick the Soldier, point blank rockets kill himself and everyone in the vicinity.
Pick the Heavy, forget to pick up ammo, run back to the base in the middle of a big fight to eat a sandvitch.
Pick the Sniper, headshot X30, then die to the enemy medic.
Pick the Scout, run ahead of everyone and die in the first couple seconds.
Pick the Demoman, blow up his entire team with sticky bombs.
Pick the Spy, face-stab the enemy heavy.
Pick the Engineer, create the most awesomely complex death machine ever, and cause the game to crash when it's fired up.
WWUD if suddenly prohibition, as mandated by the king?