My ~12 year old male garter I've had since it was a yearling just started spinning in circles over itself in its cage. It will spin for a minute or so, then go back to normal. I've seen it do this twice today. It shed last night, so maybe that has something to do with it, but I've never noticed it before. I also temporarily downsized his tank from a 40 breeder to a 10, so that could be another reason for this behavior.

This isn't my video, but this is what his behavior looks like:

https://www.youtube.com/watch?v=ISMk0jVRBfM