I just picked up a car load of darkroom from a garage sale and included was a like new thermometer, just like the one I consider to be my "trusty" thermometer.
So, I put the new one in a beaker filled with water to compare against my "trusty" thermometer.
Well, it registered about 5 degrees colder than 'ol trusty.
Okay, so I put in 3 others including a glass Delta 1 thermometer.
Guess what? 'Ol trusty is 5 degrees warm, meaning the developer temperature is really 63 when I thought it was 68.
Do yourselves a huge favor and double check your "ol trusty".