This is more of a minor annoyance than a major problem, but here goes - I'll have a program open full-screen in Windows then I'll switch to one of the other apps running on the Mac for a while, then when I switch back to Parallels, sometimes the program window will remain full-screen, other times it will quickly shrink to a little over half its previous size. The main desktop size does NOT change. I just click the "restore" button in the upper right corner of the program window and go back to what I was working on. Like I said, it isn't a major problem, merely an occasional annoyance. Anyone else seen it? OS X 10.5.6, Windows XP SP3 + all current updates, latest released version of Parallels.
A followup... I have not seen this since the latest release of Parallels and OS X 10.5.7 so I guess it was just an odd fluke.