Hey everybody! Same thing happened to my project - everything is working fine in game but in the editor sometimes spritefont is mirrored like in the first post. Looks fine when there are no effects on the object.
Also: some sprites appeared distorted a bit sometimes, looked like they were 3d planes, can't explain that unfortunately although in r170 sprites seem to be ok, still testing though!
Acer Aspire V3 laptop with Intel4000 and Nvidia GT780 video cards.