It sounds like a bug most likely.
Assuming you are using 2.8 , I would go to Blender.org and download the latest version. They are in the release candidate stage now and I think they are at RC2 . By this stage it should be about as stable as it is going to get so if your issue is a bug then the chances are it has been fixed.
Thanks.. I downloaded the latest release and that's when it started. I'll keep at it and wait for the final release.
PS: frikkr You've been super helpful answering questions lately. Thanks a ton!
It happened switching from Texture Paint Mode to Object mode in 2.8 when texturing the Treasure chest boards and metal in the tutorial. It did it intermittently so I just kept restarting until I finished.