1. Create a forum message with MDL-xxx text. On posting it becomes a link.
2. Click reply in browser with HTMLEditor, copy and paste text with MDL-xxx link in the new message (quoting). Post this message.
3. Now you could see something like
Moodle Tracker" href="http://tracker.moodle.org/browse/MDLSITE-1623">
(see http://moodle.org/mod/forum/discuss.php?d=192083#p838208 for example)
instead of link.
Other similar filters (Moodle doc links etc) may have similar problems.