As an anonymous user, edit http://prototype.wikimedia.org/de-wp/index.php?title=Wikipedia:Hauptseite&action=edit and try to join two lines by going to the end of the first line and typing the DEL key. The key seems to have no effect. Deleting other characters works fine, as does the BACKSPACE key.

Interestingly the same action works on Commons, so it's either a very recent change or caused by difference in configuration such as the CollapsibleTabs extension.

Another observation: On a local testwiki current SVN), it works fine on normal lines, joining works sometimes, but fails reliably if I try it on the following three lines:
a

b

Tested on Mozilla/5.0 (X11; U; Linux i686; de; rv:1.9.1) Gecko/20090624 Firefox/3.5, on an up-to-date Debian Lenny system.

Version: unspecified
Severity: normal