I have seen distorted views like this when using RAW+jpeg from a Canon EOS1D Mk2. When viewing RAW+jpeg I have PM prefs set to use the jpeg for the Preview not the RAW, for better speed. If I want to check the RAW in more detail I use the Q keyboard shortcut to switch to have the RAW rendered, and I have somtimes seen this stretched behaviour then. I'm not certain of the exact circumstances when it happens as to when it doesn't. My RAW+jpeg pairs are not the same pixel dimensions as each other since I use the small jpeg setting if shooting RAW+jpeg, so I'd assumed it was some sort of cache problem when PM gets confused and distorts one pic to fit the resolution of the other, or vice versa.
PM4.4.3.3
Mac OS 10.4.8