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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. 1638b3b2d3276fe5ab79eaff2464fbf7?d=mm


    Aug 10, 2017 03:15

    2017.0.1f3 here. This is a brutal issue. Even though I'm paranoid and saving constantly, I'm still occasionally losing work. It's still early in this particular project, so I'm starting to consider maybe using a different engine.

  2. Beb5f61b7198f66d0882b808affcecb5?d=mm


    Aug 10, 2017 02:33

    it happens every 3-4hours.

  3. 29011e2d5799cebf18b5190a6566f4aa?d=mm


    Aug 09, 2017 03:35

    This is happening here on my end as well, i might just have to go back to 5.6.0f3 as i was not getting this error there.

  4. F800720040398ae638193f28d28635e8?d=mm


    Aug 08, 2017 13:56

    Happens on macOS Sierra 10.12.5, Unity 2017.1.0f3. It's a big problem for me.

  5. 09b6f9f3f3f764bc39a173f48dc11dbb?d=mm


    Aug 07, 2017 11:25

    Happens here on Unity 2017.1.0f3 on Windows 8 Entreprise 64 bits.

    The Bug Report window did not show up

  6. 780a1a3fe1095c9933959882b968c211?d=mm


    Aug 07, 2017 08:38

    Happens very often in Unity 2017.1.0f3 on macOS Sierra 10.12.6

  7. 6c0b7a04056cb4cbdd4cf5980ba6bd03?d=mm


    Aug 07, 2017 03:51

    windows 10, 2017.1.0p1, comes several times in a day

  8. E97616160b76782c852d20a7b4aa23a0?d=mm


    Aug 05, 2017 01:24

    Occurring multiple times each day in 2017.1.0f3. Never saw before that. However, I am getting much better about saving my scene every time before I run. 8^)

  9. 6afbab9aa3198d4594dfd360f122e0df?d=mm


    Aug 05, 2017 00:44

    Oops, forgot the details: 2017.1.0f3 running on OsX, latest mac book pro... Seem to happen regardless of size and nature of the project I have loaded, and so far it has always happened when pressing play (I only have one instance running).

  10. 6afbab9aa3198d4594dfd360f122e0df?d=mm


    Aug 05, 2017 00:36

    Also seeing this every odd-hour randomly after upgrading to 2017.1

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