I have 26 Leupolds and smaller numbers of other brands.
Scopes almost never fail, but many think they have.
What does happen half the time is the screws clamping the base to the rifle receiver become loose.
http://en.wikipedia.org/wiki/Troubleshooting

When I go to the range I take tools and straighten out a number of rifles and help others with their problems.

When I see two holes in the target over here, followed by two holes in the target over there, I would wager 10:1 it is not turret backlash, it is loose base screws.

So if you take off the scope, tighten the base screws, put on a new scope, and it now works.... what was causing the problem? You don't know, you have an out of control variable. But most people do not have that kind of mental discipline and will think they know.

Here is an 11 second video I made showing a test for scope tracking:

There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. -Ernest Hemingway
The man who makes no mistakes does not usually make anything.-- Edward John Phelps