Last modified: 2014-04-14 20:57:50 UTC
Of course, URLs like http://meta.wikimedia.org/wiki/Special:PrefixIndex/A_ are quickly “normalized” to http://meta.wikimedia.org/wiki/Special:PrefixIndex/A by server, although formally (and intuitively) it should mean names starting not just from the letter “A”, but only from the word “A” followed by space.
But the bug lies not in URL normalization. http://meta.wikimedia.org/w/index.php?title=Special:PrefixIndex&prefix=A_ gives the same output. Even if prefix= is passed not as last argument (so _ is not a last character in the query string), then trailing space in it is silently discarded.
The same problem was already fixed for API in bug 15275
Same cause as in bug 10473.