If you look at the lid closely, it has stamped in it CAUTION RELIEVE PRESSURE BEFORE REMOVING LID
Those kegs have built in pressure reliefs in the lid that will go off when it's got too much pressure in it. When you get it off, you will see holes like vents.
To relieve the pressure, just push on the gas poppet to vent it.
On Tap: Frankenmuth Brewery's Twisted Helles
Kegged: Founders KBS Clone Brewed in '09 - might not be good anymore
Next brew: Soon I hope