Start writing a new blog post in wordpress 2.5, for example on wordpress.com. When you've finished writing it, accidentally highlight the whole text and delete it. Press CTRL+Z or Undo in your browser.
If your browser is GNOME Epiphany nothing happens, you cannot undo, you've lost your work.
If your browser is Firefox 3 you can undo. Redo, however, does not work.
In either of these browsers, on most web pages, both Undo and Redo work absolutely fine in the embedded text editor.
I am using Ubuntu 8.04 and am able to reproduce this issue in both Firefox and Epiphany by following the steps above.
I would say this is a pretty serious issue as it can lead to the loss of big chunks of user content. My friend just lost an hour's work and is very upset.