PTLens

Lenses tend to introduce a bit of distortion into your image - the more extreme the lens, the more likely it to to pull things out out of whack a bit. Fortunately, for us digital photogrpahy type folk, there's a bunch of software out there that helps us to correct this distortion.

Hoorah!

Not a problem I have yet - mostly since I'm still using the standard lens on my D70 - but I intend to have this problem as soon as I can justify the purchase of a good, wide angle lens.