After completing this checklist I am getting the following message in the console:
Has anyone else encountered this?
Does it persist when you reopen Unity and try to play your scene?
If it was a one-off I might suggest ignoring it; google-fu suggests this is an internal warning in the Unity editor code and happens sometimes with the integration between the script editor (e.g. visual studio) and Unity when saving and switching back where it compiles/runs.
That is a strange error, has it done away for you now? Were you recording your screen with OBS or similar when this happened?