In the UVW Unwrap editor, stitching a set of polygon UV's to a single poly seems to have movement randomised.
If you have a set of UV's places, and want to stitch a single polygon to it, most of the time, the whole set of UV's will move, rotate and stitch itself to the single poly!
It should always be the other way round - either weigh the amount of polys and decide which one to move (ie move the single poly to the main set), or move polys into the object that is in the centre of the UVW Unwrap window. It's a pain to have to relocate/rotate the object you're working on for the sake of a single polygon