I came across this weird tag accidentally. SO claims there are 67488 questions tagged with it. Is it a bug or a feature?
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We had a secret sub-domain that we used to test features required for facebook.stackoverflow.com and recently removed it.
This tag is just some stale data that will be purged in a few hours, after peak traffic.
I can't say why the tag was appearing in that list, but it seems to be a valid combination of wildcards. The
* character is a client-side wildcard and
~ is a server side wildcard. The server-side wildcard has the additional restriction that you need at least 4 characters in your pattern or it will be rejected.
So what this tag pattern means is
* :match anything client-side ~ :match anything server-side 294 :match 294
The last match is needed because of the server-side 4 character limit, if you remove the last character the pattern is rejected. If you use a number there that doesn't exist in any tag this part seems to be ignored entirely and you're left with a pattern that matches every question on the site.
For tag the client-side wildcard is automatically replaced by the server-side wildcard, so this pattern is equivalent to
That this URL works is probably a feature, but this tag certainly shouldn't appear in any tag listings.