Had this happen in 2017.1.0f3 a few times now, didn't get it before updating. Windows 10.

Just happened to me on Unity 2017.1.0f3... It never happened on 5.6x, so I was completely and utterly unaware of it, and just lost a night's work....

+1 from me too. Unity 2017.1.0f3 Personal on Mac.

Good luck with the bug hunt!

I've had this a few times now since upgrading to 2017 f3. I had never seen it before (I was using the most recent 5.6). Seeing as how unsaved work gets lost, it's become somewhat annoying... I have not yet observed a pattern to the cause.

I get this error about working every hour since I am using version 2017.1.0f3

I sended them this bug reported when this one was marked as fixed:

"Yep, again, this bug was present on 5.x, and it was said that was fixed, but it's not. I know, there IS another report of this exact same bug, but it's marked as FIXED, wich is absolutely not. That's a bug in the editor itself, and it's not related with the project because it happens with me independently of the project i work on, and with other people too. In previous versions it could even trigger an BSOD in Windows, wich doesn't happen anymore. It does not happen always, but it happens sometimes when switching tabs while the game is been played in the editor. In this version, the bug reporter doesn't oppen automactly, and in previous versions, unity showed an whole log of the error, in this version it just show "Fatal Error! Callback registration failed. Increase kMaxCallback.". It seems to be hapening more frequently than before. Please fix that, when it crashes, all the work made with the project except for the assets are gone. It's a huge pain."

Then they replied with that:
"Hello,

My name is Denisas and I have been investigating this case.

We are having a hard time with this bug. As it seems like it happens totally random. We have not received any reports with f1, f2 or f3, so we assumed that it has been somehow fixed.
I will reopen the 873467 case and forward it to the responsible person.
Issue tracker link:
https://issuetracker.unity3d.com/issues/unity-crashes-with-fatal-error-callback-registration-failed-kmaxcallback

Best regards,
Denisas Kirilovas
Unity QA team"

I get this error every 1-2 hour, super annoying.

Happening to me every hour or so with 2017.1.0f3. Never happened to me before 2017.1.0f3.

Also happened to me just now. Windows 10, Unity2017.1.0f3

Has happened to me 3 times today. On Unity 2017.1.0f3. I'm doing some heavy alt-tabbing, maybe it's that?