It looks like a problem with the source episode/movie. What should come in question is the format of that file (if it's either 'compressed' in a lossless codec -huffyuv, lagarith- or a lossy one -xvid or such). When editing formats with lossy codecs strange things like these can happen. It will always happen, even if you change both clips? If it does, then that would be pretty strange.
But what I meant by the second point, is that sometimes when editing in Premiere, I would accidentaly 2-click a clip inserted in the timeline, and then cutting a second clip, it would cut a part from the clip of the timeline. I would not understand some times why the hell it happened.