So I built a keezer last summer, and I'm still tweaking things. My temperature probe has been sitting on the hump, and with a noninsulated wooden collar on top there's a pretty significant temp gradient.
I've been running it at 38 degrees. Last night I decided (after drinking a couple bombers) that it would be a great idea to try to move the temperature probe from my love controller to the top of the kegs to try to cut down excess foaming.
When I moved the probe, the temp showed 48 degrees. I didn't stop to think that if the top section gets ten degrees cooler, the bottom is probably going to go down by at least that much...
I checked on it this morning. Much to my surprise, there was frost everywhere. Apparently the freezer was going all night long... the floor temperature was hovering somewhere around 20 degrees! I had one bottle in there, and the top blew off, turning the insides into beer snow (fortunately no broken glass)!
So, yeah... be careful where that temp probe is! It matters more than you might think!