Kitty can’t fix it — Old questions inaccessible for logged-in users, giving “unexpected error”

Question

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:

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:

• If you break a stick at two points chosen uniformly, the probability the three resulting sticks form a triangle is 1/4. Is there a nice proof of this?,
• Do sets with positive Lebesgue measure have same cardinality as R?,
• Chain homotopy: Why du+ud and not du+vd?,
• Ordinals that are not sets,
• Important results that use infinite-dimensional manifolds?,
• Why is addition of observables in quantum mechanics commutative?,
• Is it possible to dissect a disk into congruent pieces, so that a neighborhood of the origin is contained within a single piece?,
• Is there an introduction to probability theory from a structuralist/categorical perspective?,
• Polynomial bijection from $\mathbb Q\times\mathbb Q$ to $\mathbb Q$?,
• Fermat numbers and the infinitude of primes,
• Representations of Pin vs. Representations of Clifford,
• Convergence of $\sum(n^3\sin^2n)^{-1}$,
• Is Thompson's Group F amenable?,
• Is the Riemann Hypothesis equivalent to a $\Pi_1$ sentence?,
• Fast matrix multiplication,
• Is all ordinary mathematics contained in high school mathematics?,
• How to find the number of $k$-permutations of $n$ objects with $x$ types, and $r_1, r_2, r_3, \cdots , r_x$ = the number of each type of object?,
• Demonstrating that rigour is important (Note: only page 2 is problematic),
• Awfully sophisticated proof for simple facts,
• Examples of ZFC theorems proved via forcing ,
• Arguments against large cardinals,
• Proofs of the uncountability of the reals,
• Does "antichain" mean something different in set-forcing than in lattice theory?,
• Cantor's argument revisited,
• Nonessential use of large cardinals,
• Behaviour of power series on their circle of convergence,
• Axiom of Symmetry, aka Freiling's argument against CH,
• Splitting infinite sets,
• Inconsistent theory with long contradiction,
• Collecting proofs that finite multiplicative subgroups of fields are cyclic,
• Slaman and Woodin on Mathematical logic,
• The disjunction property in Peano Arithmetic?,
• Algebraic axiomatization for AB+BA^T operation on matrices,
• Exact consistency-strength of "all projective sets are Ramsey",
• Fifth powers modulo a prime,
• Does $2^X=2^Y\Rightarrow |X|=|Y|$ imply the axiom of choice?,
• Intersection of an uncountable number of sets.,
• Solovay's paper from AD+ that all sets are Ramsey,
• If d/dx is an operator, on what does it operate?,
• How are infinite-dimensional manifolds most commonly treated?,
• Consistency strength of projective determinacy (PD).

Answer 1

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.

Answer 2

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.