11. You can build Sentries underwater. If you have a Dispenser next to you, it'll heal you faster than you lose health from drowning.
12. Disguised Spies appear injured to the enemy team. If an enemy Medic heals them, their apparent health will go up. But if they stand near an Engineer's dispenser, the healing beam will activate but their health will stay the same. Look out for this.
13. A Medic can Ubercharge two players at once by switching between them very rapidly with his medigun. It lingers slightly on each person after you switch, so if you can switch back and forth quick enough, all three of you will be completely invincible.
14. A cloaked spy becomes visible briefly if hit by enemy fire, or touched by an enemy player.
15. There's a corner of the water on 2fort, on the Blue side, where it's shallow enough to stand. It's a great Sniper spot, and Soldiers can use it to rocket-jump up onto dry land.