I was making English toffee last night, and there are telltale signs of when it is nearing being done, like puffs of steam escaping. So I keep watching the thermometer, and it sort of stalled around 225, but yet the toffee was definitely getting hotter. Finally when it was huffing and puffing, I did the old fashioned water test - dropping a bit in a glass of cold water, and sure enough it hardened instantly - yet the thermometer still was only at 225.
The only thing I can think is that there was condensation inside and that was throwing it off. Regardless, I bought a new thermometer today. I've got the cap off of the other one, and I'm just going to let it dry out for a long time.