Fixed in r26315. Note, though, that for the result you intended you'll still need

http://en.wikipedia.org/w/api.php?action=query&prop=revisions&titles=Moravian%20Cemetery,%20Staten%20Island&rvstartid=155640991

or

http://en.wikipedia.org/w/api.php?action=query&prop=revisions&titles=Moravian%20Cemetery,%20Staten%20Island&rvendid=155640991&rvdir=newer

r26315 seems not to have fixed it, or at least backwards, now rvendid seems to work as rvstartid, and rvstartid works as rvendid, or is it some backward logic here I've failed to understand?

If my logic, creating queries, are wrong, perhaps you can enlighten me, this was only a part of the query I want to use, the original is http://en.wikipedia.org/w/api.php?action=query&prop=revisions&titles=Moravian%20Cemetery,%20Staten%20Island&rvlimit=1&rvendid=155640991&rvuser=Arbus%20Driver
I want to grab the latest revision, if any, that is made by a specific user with an revid of specified or lower (defined in my script by current revid minus one).

The logic is present. If you have rvdir=older, revisions 1-5 will be listed in the order 5, 4, 3, 2, 1. If you want that list to END at 3, that means 5, 4, 3. What you want is 3, 2, 1, a list STARTing at 3. Of course you can also reverse the order (rvdir=newer), which will give you 1, 2, 3, 4, 5; 1, 2, 3 and 3, 4, 5 respectively.