Yesterday I made a double-batch of fudge. Nothing fancy, just the recipe on the back of the Marshmellow Creme jar. It calls for cooking the first three ingredients to 234 degrees, which I did, according to what my usually-trusty candy thermometer read. The fudge turned out okay, but a little dry.
Today I made another double-batch of fudge. Same recipe, same ingredients, same method, same pot, etc., but I decided to test my thermometer first this time. I put it in a pot of boiling water, and what do you know... the water was at a rolling boil at 208 degrees! Water, as you know, boils at 212 degrees, so my thermometer is 4 degrees off.
If you haven't done a lot of candy making you might not know it, but four degrees can make a big difference! So with today's fudge, I took it off the heat at 230 degrees to account for the difference. And what a difference it made! Today's fudge is more creamy and just an overall much better consistency. Not dry like yesterday's, and although the flavor should theoretically be the same, it really seems to taste better. (Humidity also plays a big factor in candy-making, but I don't think there was much difference in humidity between yesterday and today here in Arizona.)
So, even if your candy thermometer has been faithfully accurate for you in the past, it's a good idea to test it before using it... especially when you're making a large batch of goodies to give away to your friends :)