Just had an ugly experience -
Had been trusting the thermometer on my first SW tank for the past 6-8 weeks - seemed warm, but I assumed that was normal.
Had a few fish die unexpectedly, even though the other water parameters seemed correct, I had never checked the temp. Today, the tank felt noticably cooler, and I reset the heater by unplugging it / plugging it in, and it quickly heated up - but, that inspired me to go grab a heater.
Grabbed a glass thermometer first, and it read 88 - way too hot, even though the tank felt colder than usual. Grabbed a quick digital thermometer that read 89.2 - wow. Realized that the heater was still on despite it being set to 79 - clearly the thermostat was stuck.
The tank's been at 89-90 degrees for 6-8 weeks - I'm actually amazed that 'most' everything was alive. The tank's had chromis and crabs since the cycle was established, living near 90 degrees without many problems at all. A sixline wrasse didn't make it, and I lost a few snails, but the chromis and hermits must be amazingly strong.
Taking steps to lower it now - swapped out the header, floating an ice bag in the sump to lower it slowly, have a glass thermometer on the front glass so I can monitor it much more regularly.
Still really depressing that it let it go this long without ever checking with a reliable thermometer.