Just made a batch of strawberry jam, same recipe I've always used, but decided to use my fairly new candy thermometer to bring it up to the temp of 215 the recipe recommends. Usually I just bring the mixture to a boil, and let it go for about 15 minutes, then use the cold plate test to see if it's set.
Well - the thermometer went up past 220 and the jam still wasn't setting, so I just went ahead and did it the way I usually do. Might explain why the fudge I made at Christmas didn't set - was going by the thermometer instead of my eyes!
I also have a Kitchenaid oven thermometer, and have noticed that it runs about 20 degrees off from the oven temp.
So - when you buy a thermometer, how do you trust it to be accurate? And how can you check the accuracy?