Well, yeah, I have "heard" of them before, but I would think there is usually always a pretty good reason for it happening, and is probably caused by something the keeper isn't really aware of. Rather than it just happening for seemingly no real reason right out of the blue. Know what I mean?
Hard to prove or disprove though unless you know for a fact it wasn't caused by one of the other things I mentioned.
In my personal experience, I have only noticed this ever happening with any of mine when there was a previous shed that was never sloughed off in the drier winter months, albeit an extremely rare occurrence.
~Doug
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"a snake in the grass is a GOOD thing" 
my website -Serpentine Specialties