Steps to reproduce:

1. Go to (say) https://en.wikipedia.org/w/index.php?title=Visual_editor&veaction=edit

2. Place the cursor after the end of a paragraph (e.g. after the full stop in "editors are.")

3. Hit the delete key to merge the paragraph with the subsequent one.

4. Type any character (a letter, blank, etc.)

Result: Chaos ;) - large swaths of text are deleted, and (apparently) random characters appear near the beginning of the page.

Reproduced in Chromium (Version 33.0.1750.152) and Firefox (28.0).

Version: unspecified
Severity: major
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=62333