This happened in Blender 2.65 and 2.68 for me. My UV maps are being warped and I see no reason for it. I had a mirrored plane (flying plane), and one side of the plane (in the uv map) was oddly warped:
(Using Smart UV)
Thanks
For the crate what unwrap method were you using. For non norganic objects you may get better results with Conformal instead of the default Angle Based (change in unwrap options in toolshelf). The results will also depend on you having correct seam placement
For the plane don’t be lazy with Smart unwrap. Use proper seam placement so you have control of the unwrap. Also look to use Ctrl+A to average the scale of the islands
I forgot to mark some seams in the crate. So that’s okay now. I made the plane for a game, I didn’t want to spend too much time on the models as I prefer programming to modeling and such.