If a larger snake happens to pee over the probe, it will get wet and results in the probe reading a cooler temperature and thus turns on the heat mat beyond where it should be. This can result in burns, especially if the mess isn't cleaned up quickly.
Many people do it inside and are OK, but many of us here are not willing to take that risk with our animals, no matter how small that risk may appear. Many people can't afford a vet for a burn when an extra 2 minutes to put the probe outside can prevent any chance of up to hundreds of dollars of vet care.
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