The sequence above was performed on Firefox on Win7.

I've tried reproducing this on Firefox 25 on Ubuntu 12.04. Errors occur at all 3 locations for which errors have been described:

The position of the cursor after hitting delete is unpredictable (read: Deryck has seen a few different outcomes and doesn't have enough time to exhaust all the possibilities); occasionally it even goes to the correct place.

The "eating up" of the next (sometimes even previous) paragraph only occurs if you start inserting text immediately after hitting delete. If you click anywhere in the textbox or hit the arrow keys on the keyboard, no text is corrupted.

Hitting Ctrl+Z after these errors yields unpredictable behaviour (same meaning of "unpredictable").

Similar, scary things happen at fr.wp, as reported by VEditors there.

Just a few examples:
0. Try to VEdit https://fr.wikipedia.org/wiki/Utilisateur:Elitre_(WMF)/Prove .

1. Hit Del right next to "blabla".
2. Notice the cursor is now on the top left margin of the page.
3. Hit any key, say "w". I now have two more lines in my section, the first with a "b" on it, the second with a "w".
4. Wait for some seconds. Watch as a "b" gets magically added to the header title.

0. VEdit the page again.

1. Hit Del right next to "Hawker Siddeley Andover".
2. Hit any key, say "w". Watch as a pawn appears on the previous line, and both the letter and the pawn multiply after a few seconds.

Another example:

In https://en.wikipedia.org/w/index.php?oldid=616866203&veaction=edit , place the cursor left of "F" in the word "Former" in the first paragrah, and hit the delete key. The "F" is deleted correctly, but the cursor jumps two to the left (between "s" and ".").

Reproduced in Firefox and Chromium under Ubuntu 14.04.

The original issue(s) appear(s) to be fixed. The issue spotted by Tilman has been filed as bug 67985.