Some permalinks to specific question revisions go to 404

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While reviewing 404 errors in my GWT account, I came across a number of odd-looking 404s:

• answers/54/revisions/ (linked from http://expats.askbot.com/answers/34/revisions/)
• answers/48/revisions/ (linked from http://expats.askbot.com/questions/27/revisions/)
• answers/39/revisions/ (linked from http://expats.askbot.com/questions/22/revisions/)
• answers/31/revisions/ (linked from http://expats.askbot.com/questions/13/revisions/)

And so on.

Here's a screenshot from http://expats.askbot.com/questions/13/revisions/ indicating where the broken link appears:

Note that the other revision permalinks do work; it's just the second one that doesn't. The other pages in the above list have similar behavior.