For the past few days, any attempt to access the question Collecting proofs that finite multiplicative subgroups of fields are cyclic results in an internal server error:

screenshot: Oops! Something Bad Happened!

It seems the poor kitten needs help from a human.

The front page indicates some activity on this question meanwhile, hence it cannot be broken for everyone. Moreover, I checked that I can access the question when I log out, but after I log back in, it’s broken again. This happens consistently across multiple devices (albeit on the same network).

The associated timeline and revisions pages work fine.

The same problem affects many more questions. As observed by Sam Hopkins, they all seem to be $\ge10$ years old. Martin Sleziak found out an important distinguishing feature that all affected question have some deleted answers. Moreover, evidence suggests that the bug only happens for users with $\ge10$k reputation (possibly because they can see deleted answers).

It appears from quick sampling that about 2/3 of all old questions with deleted answers are affected by this bug, which means it’s likely several thousands of questions.

As for the cut-off date, the last question affected by the bug that I found is Computing Ext in Exterior algebra (related to Koszul duality) from June 24, 2013, which is the day before the integration of MathOverflow into the Stack Exchange network. Thus, it seems likely that the bug only affects questions from before the migration.

Based on further experiments, it appears a more precise criterion is that the bug affects questions that have a deleted answer that was deleted by June 24, 2013, except when the answer was deleted in a special way (e.g., here the answer was converted to a comment; here it was deleted by spam flags).

A small sample of questions exhibiting the problem as reported in the comments:


2 Answers 2


Thank you all for reporting and excellent documentation of this issue! We have applied a fix and all the affected pages should now load as expected for logged in users as well as anonymous users.

As you all observed, the issue was indeed linked to older pre-migration post history deletion data which was formatted differently than it is today. Some code was released recently which did not handle the old history format, resulting in the error. This impacted both questions and answers but only in some conditions as you identified, i.e. the page would load if the post was flagged as spam/offensive.

The fix that was applied reformatted the old data so that we no longer need to be concerned with special handling, hopefully preventing headaches like this in the future.

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    $\begingroup$ Many thanks for this! $\endgroup$ Mar 3 at 17:41
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    $\begingroup$ Thank you for fixing it! $\endgroup$ Mar 3 at 17:57
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    $\begingroup$ Thank you for this! Who would have thought the migration would still make itself felt this much later?! $\endgroup$
    – David Roberts Mod
    Mar 6 at 9:48

This is not a resolution of the bug, but at least I have found a way around it: If I log out of my account the affected pages all become accessible. If I then log back in I get the "kitty" error, but logging out makes the page accessible again.

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    $\begingroup$ Yes, this is noted in the question. $\endgroup$ Feb 20 at 11:20
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    $\begingroup$ ah, apologies, I had not read carefully, this answer then does not add anything, I will delete it. $\endgroup$ Feb 20 at 11:24
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    $\begingroup$ Note: Instead of logging out, you can open an incognito tab on whatever browsers support this. $\endgroup$ Feb 27 at 15:30
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    $\begingroup$ This is an okay temporary fix, but I really hope someone (at StackExchange support, I guess?) is aware of the bug and working on a real solution, because this is a big loss in functionality and has been going on a while now. $\endgroup$ Mar 1 at 2:13

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