Steps to reproduce

1. Create a 2x2 table
2. Enter "a & b" (with spaces) in lower left cell
3. Enter "a&b" (without spaces) in lower right cell
4. Mark and copy " & " (with spaces) from left cell
5. Highlight "&" from right cell and paste from clipboard.

Editor locks up, operations such as newline (using the Return key) and paste from clipboard still works, but user will not be able to add any letters by typing them on keyboard.

Video on how you can reproduce this: http://screencast.com/t/W5inrgmyQjEl

This is reproducible on Firefox 8 and 4.0.5. 4.0.3 seems to work fine though.