I've had a stick-on LCD (thermochromic) style thermometer since my tank was up and running, and I've always been dubious about it's accuracy. I've recently noticed in the cold weather that the tank temp would be two deg down in the morning, and on Saturday, when it became really hot while we were out and the lounge had heated up, I noticed the tank temp had gone up by a degree. So I put the usual alcohol-in-glass style from my isolation tank in the main tank and that's kept a nice stable 24
oC for the past couple of days.
I'm sure that the Thermochromic thermometer is influence by the room temp as it's stuck on the outside of the tank. I've binned mine and now I have an excuse to go to the LFS again.
Anybody else noticed this, care to experiment, or just a heads-up to keep an eye out. This would explain why, in those few really hot days we had last summer, I thought my fish were going to poach. They were fine.