This bug has been observed for a while on fr.wp and thus unofficially reported time ago, but now User:Drongou has kindly provided instructions to replicate it, so:

1. Copy/paste something between square brackets, say [[2013]]
2. Place the cursor before the 3 and type 4, so that it becomes [[20143]]
3. Type a space, so it now looks like [[2014 3]]
4. Remove the 3 and save.

The result will be <nowiki>[[2014 ]]</nowiki>.
Confirmed at least on Safari 7.1, Mac OS-X 10.9 .

Version: unspecified
Severity: normal