I've just found the solution to an annoying problem. When I edited some horizontally placed Mtext it's rotation would automatically change to 270 degrees. On finishing editing it would remain at 270 degrees and I would have to be set it back manually to 0 degrees.
Looking into the problem suggested a number of avenues of investigation, most of which did not lead to the solution I was looking for. They included the "mtexted" variable, oblique angle text settings and the UCS not being set to "world".
In the end I found the rather obscure solution on another forum and though I would repost here for prosperity.
It turns out that I was using the @Arial unicode font in a standard text style. When I reset this to plain old Arial (without the @) the issue disappeared.
Hopes that helps someone.
Regards,