i was making some alignment marks the other day and things weren't quite lining up, so I pulled out a 4' straightedge, and touched each end to the edge of the printed area:


Kind of hard to see, all I had was my phone. There center of the topsheet was bowed about 3/16-1/4 inch, while the plastic itself was lying perfectly flat. This makes me think it was something during printing, opposed to dimensional instability of the material in my shop.
Oh, and this is what I was working on:

I was able to cheat the really bad side and keep most of the graphic on the ski. It would have been better had my customer left space around the edges, from now on I am making my graphic masks that I send out 2cm smaller then the actual ski dims.
I won't say who printed it, but there are only 2 companies I know of who do this on a small scale basis. And the last topsheet I had printed by the other one didn't line up too well either, but I did not confirm the warping.