having the cross hairs not level.

All the information above is good advise but I would have to add. Rushing to install a scope is bad news. Taking time insures that the job will be well done with care of course.

Adjusting eye relief wearing just a Tshirt instead of your hunting coat.

gotta agree with gopher, ive seen so many guys hand me their gun, and my first comment is "the cross hairs are crooked" and they say "No their not", its because they line them up to be square when their head is laying over the gun and not square to the gun.

[:@]getting in to big of a rush,take your time and do it right

If using a 1" scope tube, get a 1" wooden dowl rod, cut to legnth and align rings until the dowl fits properly. Especially important with the Leupold twist lock bases. Not the best method but if your going to dot he job at home it is cheap and fairly effective, will keep you from crimping your scope tube.