There are different variables. Altitude, water hardness and other things will affect the boiling point of water, so if your therm is a few degrees off, it isn't something to worry about.
Water can get hotter than 212. It's called super heating. But it will only happen under pressure. A pressure cooker is a prime example of super heating water. There are steam rockets that have entered outer space that use water super heated under pressure then released through a nozzle, flashing the super heated water instantly into steam providing thrust to propel the object. In fact, one of the worlds quickest rocket powered dragsters is propelled by a steam rocket motor, using this method. This is also the way a steam locomotive or tractor works. By retarding the flash point of water to steam under pressure, we can harness thrust from water and convert to horsepower.
Chad, my brother is a refrigeration guy, he did it for 3 years in the Navy and has done it as a civilian for about 15 years. They use ice water to calibrate their therms. Since ice can't get below 32 degrees it is a valid way to check the accuracy of the therm.
The most accurate way of calibrating is to either boil or freeze distilled water. If boiling, you have to adjust for altitude, but the difference is so minuscule, it doesn't really matter for our purposes.
If anyone is interested in steam rockets, I've built several and have done extensive research. I have built rocket motors using water for steam and also hydrogen peroxide. I used to be heavy into 1/10 scale radio controlled dragsters. And not the little kids Tycos or Radio Shack stuff. Big buck, big boy cars that are insanely fast. I currently have the worlds quickest radio controlled dragster, and get no recognition from the only R/C dragster sanctioning body, because I pi$$ed off the president. Under a normal situation, a president could be voted out. But years ago when he founded the organization, he claimed ownership and set himself up as permanent president. Sounds more like a king to me, but that's life I guess.