These four diffs all show the error in progress over the past couple of days to
different editors. There are also several other occasions where this has
happened; these diffs all have one thing in common - the editors have apologised
and expressed surprise, proving that this is not vandalism. Other occurances of
it cause vandalism warnings to be issued to both registered and anon users, but
it's not possible to tell for sure unless the editor in question retorts.
This same error has happened to me in the main article space of en 'pedia, and
on my user and talk pages, and to a template page, but after the first time it
happened I've started to check the bottom of the edit window before saving to
ensure the text is present.
I'm assuming "critial" status applies as this is causing data loss (albeit
/revertable/ data loss).
OS: Windows XP