If you select some text whose first word is bold, and attempt to make it all bold, it will look as though it has succeeded, but actually too many ' characters have been added, so it will appear as bold, italic and single-quoted, when saved!
Note: my selected text also included an internal link - that might have been relevant.
Version: unspecified
Severity: normal
OS: Linux
Platform: PC
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=42803