Some questions show a tag that nobody put there

Question

I ran into this post (74102654 on SO) as a review audit. It has the dart tag, though it's not related to Dart. I was about to comment about it, but thought to check its revision history (which at the time of this writing is at revision #6) to see if it was some kind of spam/vandalism edit first. From what I can see, the dart tag has never been in any of the revision history, including the first revision.

According to its Q&A page, its tags are: dart c++ compiler-optimization undefined-behavior integer-overflow,
but according to revision #6, which edited tags, the tags are: c++ compiler-optimization undefined-behavior integer-overflow integer-arithmetic.

I don't have the privilege that allows quick tag edits, but when I edit the whole post, the bug manifests (shows dart tag instead of integer-arithmetic tag).

The dart tag also doesn't exist anywhere with the post according to SEDE (reusing a query written by @rene).

Here's a screenshot of the Q&A page, and here's a wayback machine link.

@Zoe gave a look from her end and saw the same thing as me. She suggested it could be due to a bad case of caching. Revision #6 was made on Oct 20th, almost three weeks ago from the date of this writing.

Some SEDE Queries of SO

• Posts with no revisions and probably-bug tags
  • Those which have had a completed review
• Posts whose latest revision is a tag edit that doesn't match the post tags
  • A similar query that isn't limited to latest-revisions, but might have some false positives from approved suggested edits.

You can try out the same queries for other network sites. Note: I'm new to SQL so if you want to give me feedback in the SEDE chat room that'd be much appreciated. I've been trying to write a single query that combines most of the above, but am having trouble with the SEDE timeout.

Comparisons with similar bug reports

• There is this very similar bug report on MSO (421265).

  • Both 421265 (on Oct 24th) and 74102654 were used as review audits.
  • 421265 became a hot network question on Oct 21 at 21:36 and got a tag-only edit on Oct 22 at 20:34 (by a 10k+ user), and 74102654 became a hot network question on Oct 18 at 4:09 got a tag-only edit on Oct 20 at 15:41 (not a 10k+ user).
  • Looking at the wayback machine, 421265's bug manifested between Oct 24 and Oct 26. For 74102654, it manifested between Oct 27th, and Nov 9.
  • There is a difference in what is observed in SEDE for the 421265. See this one of its MSO post's answer for more details. But this difference might just be that the bug is related to audits, and since I did my audit today on the same diay I did my SEDE query, SEDE is just not updated yet.

• "Spurious unkillable tag", this (reported by a <3k user), and this (reported by a >3k user) say that the "ghost tag" doesn't show up in the post editor UI for the bug reporter, but in the case of the post in question here, the "ghost tag" does show up for me in the post editor UI.

• Probably unrelated bug that got fixed: "Tags added when improving an edit become unremovable". Doesn't seem very related since I don't see evidence that the post in question here had an improve-edit review (no consecutive edits with almost-same timestamps).

Answer 1

I wrote an SEDE query to try and find more occurrences of those "ghost tags" and got some very strange results. The query is not so efficient, so I limited it to 100 results.

Unfortunately, many of these "ghost tags" appear to have been introduced by a rollback for some reason (even though the tag doesn't seem to have ever existed in the question before the rollback). This behavior might be bydesign and/or is a side effect of some operations performed on the those posts (by staff?), so that might need to be discussed in a separate post. Therefore, here's another version of the query excluding those that were caused by a rollback.

Post	Tag	Note
Compatible encryption between C# and PHP, ColdFusion, Ruby, Python	scala
C# Dynamically created LinkButton Command Event Handler	asp.net	tag existed as asp.net.
What is the best XSLT engine for Perl?	xslt
ListBox elements rearranged with JavaScript causing event validation error on postback	eventvalidation	tag existed as eventvalidation.
XSLT Transform Efficiency	xslt
Vanishing scroll bar when using Autocomplete extender	ajaxcontroltoolkit

I tried to execute the query on other sites to see if the problem has occurred elsewhere, and it seems to indeed have. For example.

• On Super User, there's a lot of occurrences with the windows-xp tag.
• On MSE, it happened with 3 posts, two of which don't have a complete revision history and the third one doesn't show any tags under /revisions.