My last 3 batches have turned out very poorly -- and I think I've discovered why. A while back I found that my floating thermometer had broken, so I had to use a digital thermometer instead.
Even though I had checked the digital against boiling water and in the freezer, I still didn't trust it, but I had no option. Since then I acquired 2 new thermometers, and -- compared to them -- it reads high by about 9 degrees in the mash temperature zone, which means I mashed way too low, ruining my starch-to-sugar conversion and changing the profile of the beer. (These were beers I have successfully brewed before -- with the floating thermometer.)
It's enough to piss off the Good Humor Man, and I'm not the Good Humor Man!
The take away is: check your thermometers and have reliable backups.