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Fixed in 2018.3

Fixed in 5.5, 5.6, 2017.1, 2017.2, 2018.1, 2018.2



Found in



Issue ID




Unity throws out Fatal Error: Callback registration failed kMaxCallback



Unity throws out Fatal Error: "Callback registration failed kMaxCallback" after working with the project for 30 minutes or more. It appears randomly.

1. Actively use Unity for more than 30 minutes

Actual: Unity continues to work
Result: Unity crashes with Fatal Error.

Reproducible with: 2017.1.4f1, 2017.2.3p1, 2017.4.4f1, 2018.1.3f1, 2018.2.0b6

Reproduced on macOS, Windows, Linux.
Bug Report window does not appear.
Scene files are not restored after reopening Unity so all not saved Scenes are lost.

  1. Response avatar


    Jan 04, 2019

    Please file a new bug report If you are still encountering this problem. The error message is a symptom and can be caused by many different areas. The area for this bug has been fixed.

Comments (207)

  1. Ca00610c0581ae76c4e531a544cbe684?d=mm


    Sep 03, 2017 11:13

    Patch 2017.1.0p5
    Same here.

  2. 7181c6fca09bd7cb8d227a4b8250dd43?d=mm


    Aug 31, 2017 14:48

    They fixed the problem in 2017.2.0.b9 (Beta)!!!

  3. C5035e6f9ac6c7ee6b43b629d5ec96ea?d=mm


    Aug 30, 2017 16:32

    unity exits automatically many times a day.

  4. 8c126b2f288bc3a5cbd76422c7f97077?d=mm


    Aug 28, 2017 03:27

    I just installed Unity today version 2017.1.0p5. I have started watching some tutorials on the site and going through them and appears this happens randomly anytime I alt tab between Unity and Visual Studio. It can happen every 30 minutes to as soon as I start Unity back up and alt tab for the first time.

  5. 22779d66753cc4af08a34a461ce9978a?d=mm


    Aug 27, 2017 17:39

    Now it is common for me. It happens at least 6 times a day to me. And I have already lost many hours of work. I have Unity 2017.1. Fix it please!!!

  6. B73bcdee47ea96e48ea9fe349cc61ada?d=mm


    Aug 26, 2017 23:20

    A dozen crashes a day with this error on MacOS with latest version of Unity. Praying for a fix.

  7. 2ecf0c3c4065a1a1fbf18f32275190fe?d=mm


    Aug 25, 2017 08:58

    this is happening to me now around 5 times per day! Very frustrating. Lost some work a few times now.


  8. A426725f19002fd2bbb88ed2d43d1a9e?d=mm


    Aug 23, 2017 10:25

    Most annoying bug! I almost destroyed my keyboard because of that ;) ... Still happening on Unity 2017.1.0p4(p3)

  9. B89ae67e47d974f5a427480148083a89?d=mm


    Aug 20, 2017 18:51

    Ouch, save often and always before play mode! My scripts were not erased, but my editor creations were. Sadness is overwhelming. =P

  10. Dbd49e53586c7ee10350caa4e5220f76?d=mm


    Aug 20, 2017 13:22

    Please, please release an urgent fix for this bug! I get this up to 10 times a day working on projects! Occurs mostly switching between monoDevelop and Unity Editor often during debug sessions.

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