Oof. I feel your pain. I had a thermometer problem on a batch this summer. I wanted to use one of those infrared "laser" type of thermometers. The evening before brew day, while cleaning my brew kettle, I checked it against two other thermometers: the one installed in the kettle and my old buoyant mercury thermometer. Everything seemed to check out, and I thought man, this infrared thermometer is gonna be AWESOME.
The next AM, I got the ball rolling and started heating up the strike water. Monitoring my water temperature with the infrared thermometer, I hit my strike temp (or so I thought), mixed the water with the grain, and watched in horror as the mash temp rocketed past the 150 degree F. temp I was shooting for and into the 170+ range. I scrambled to add enough cool water to get back into the 150 degree ballpark and it worked, but man, that is not the way you want your brew day to start. Ugh. I bought a lab grade traditional thermometer the next day, and that's what I'm sticking with...