6. Even without a Medic, a Heavy will win a firefight with a fully-upgraded Sentry if he's close enough to it. Don't place your Sentries too close to entry points.
7. The Heavy's fists are the only melee weapon with a secondary fire. Left mouse is a left hook, right mouse is a right hook, and critical hits are uppercuts.
8. A Sentry gun can fire in any direction, but takes a moment to turn around if attacking an enemy behind it's normal arc of fire. You can place them closer to walls if you right-click twice before deploying - that rotates the turret's position to face you.
9. If he gets in its way, an Engineer can be killed by his own Sentry. It won't do damage to team-mates, though, even if they stand in its line of fire.
10. Players always emerge from teleporters facing the same direction. Instead of destroying an enemy teleporter, stand behind it and kill the players as they come through. Particularly effective as a Spy.