It's affecting Firefox 4 and IE9 as well. In fact, it almost got me a few minutes ago...
I was posting this response in the new How To forum:
I had copied-and-pasted the entire text from one Firefox 4 window to another. I saw it strip the formatting (another bug), so I hit Ctrl + Z and then disabled the rich text editor to paste again. The text field should have been completely empty, but here's what I got:
I erased that text, pasted in my own, and switched back to the rich text editor. It then insisted on appending <span id="INSERTION_MARKER"> (this time without the ) to the very end of my post, no matter how many times I removed it. It persisted for as long as I used the rich text editor until I refreshed the page and started over, pasting in my text again w/o the rich text editor being enabled.
I have also reproduced it:
1.) Without copying-and-pasting text - I just typed in the rich text editor normally and used Ctrl + Z.
2.) Without using any shortcut keys - I used the right-click menu to "undo" instead.
If you follow the following steps, you can reproduce it every time in multiple browsers.
1.) Copy-and-paste text into the body field with the rich text editor enabled.
2.) Undo the paste operation.
3.) Disable the rich text editor.
Hopefully that makes it a little easier for the developers to reproduce it for testing/debugging purposes.
1.) I have not seen this issue yet without using "undo" in some way. Has anyone else considered their usage of "undo" in reproducing this issue?
2.) The insertion marker is visible after posting, when previewing, and after disabling the rich text editor.
3.) While using the rich text editor, you can see when this happens - an extra space is added that you never typed. It represents this "insertion marker."
4.) Since this can happen in any browser, you'd need to disable the rich text editor entirely - there's gotta be a better way / a permanent solution.