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$wgPasswordAttemptThrottle doesn't expire when using xcache.
I tested with versions 1.19.8, 1.19.9, 1.20.8, 1.21.3 and git 1.23alpha (c232652) 2013/12/04

includes/specials/SpecialUserlogin.php
function incLoginThrottle()

$wgMemc->incr( $throttleKey );
// incr() replaces expire ttl with 0, so it never expire.

// it can be fixed replacing incr() with
$wgMemc->set( $throttleKey, $throttleCount+1, $period );