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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 (206)

  1. F27617e362df1b431c68e2cbc49040bf?d=mm


    Aug 14, 2017 18:23

    Mac, 2017.0.1f3 - Please let me know if I can help repro, I get it several times a day and it's very hurtful to our progress.

  2. 854e499507e78aa82deb1c6a8858435c?d=mm


    Aug 13, 2017 22:11

    God! Please friggin fix it! We are losing progress and need to reset scenes!!!
    The hell?! 2017.1.0p3 and still happening.

  3. 40d9242731643561050862cdb8f66f2b?d=mm


    Aug 13, 2017 17:11

    This is clearly happening to a large group of people and is certainly a hefty annoyance. Can we get some official Unity word on what the plan is for this? If this issue was any more frequent it would be a show stopper.

  4. Ea7a78df7acb9ead19af66d6199a1bbf?d=mm


    Aug 13, 2017 16:19

    Happens always after a period of time (~half an hour) when switching from Visual Studio to Unity and hitting play. I have not experienced error to show up when using JetBrains Rider yet.

  5. D02f347f4985918306796bff3bb47ded?d=mm


    Aug 13, 2017 09:47

    Also just happened today with me - 2017.1.0f3 latest VSCode, mac. Had been working fine for hours, first time it's happened.

  6. 6539f4c005d74696be9ae3b2c6a1267b?d=mm


    Aug 12, 2017 00:50

    Happened to me twice today loosing 30 minutes of work each time. Only one instance of Unity Open and when I click Play :(

  7. 4e9828e48c70f0896dc60863372156d2?d=mm


    Aug 11, 2017 22:44

    Please fix this fast, crashes 3-7 times a day.

  8. 64e0a502c7d6020a2825f27141db3719?d=mm


    Aug 11, 2017 19:25

    It happens sporadically in Unity 2017.1.0f3. I was left with the feeling that is caused in conjunction with VS 2017.

  9. 8f5913f71c236e2f0959309b3e5edc52?d=mm


    Aug 11, 2017 09:44

    Same issue for me. Happens irregularly, but typically each 2-4 hour into my work. No data is saved on crash. Typically happens when pressing play but cannot confirm that for every case.

    Unity version: 2017.1.0f3

    OS: Windows 10, 64-bit.

  10. Ff7f90c707b31106f4f27da733ea0a7c?d=mm


    Aug 11, 2017 02:28

    Unity 2017.1.0f3 running on Win7 Pro 64 bit version and the error pops up about every hour. It makes development a little more stressful wondering what I'm going to loose and when.

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