Crashing after warning of Invalid magic of next frame

I can't seem to find a way to reproduce it - seems to happen randomly during fit tests.   Not like a specific test will show this behavior repeatably. The frame that is printed is different each time, as is the bad magic.   

So – unsure how to approach this - but it seems to happen when there is a lot of activity – a busy client (of course, this is usually in transactions).   Probably worth TSAN during fit tests, and do that multiple times perhaps...