I've had a few scopes go south on me over the years. Had a few cheap Tasco's on air rifles as a kid that had the crosshairs come loose and rather than being oriented like a + they rotated and were more like an X and would rotate to a different position every time I fired a shot.

Around the 2006-07 timeframe I had a Mueller scope that had a seal go bad. What was a good clear optic one deer season turned into a very milky or soap scummy picture when looking through when I pulled it out of the safe the following hunting season. Basically a seal failed and allowed oxygen/moisture into the internals. This type of failure isn't nearly as common today as it was back in the earlier days of optics as we have far better manufacturing methods and the seals are made of much more durable materials that are far less prone to drying out and cracking over time.

About 10-11 years ago I had a Weaver Classic V 6-24x that stopped holding zero after about 3 years. Gun went from being a 3/4" or better to shooting 4-5" patterns at 100 yards which is what raised my suspicions. This is probably the most common failure most will encounter, and usually has to do with a weakened or defective erector spring which is the spring that keeps tension on the internal tube in the scope opposite your turrets. Most are located at the 7 o`clock position when viewed from the rear of the scope and push the erector assembly up and sideways into the windage and elevation turret abutments. Insufficient tension on this spring allows the erector assembly to jump and move around a bit from the recoil causing the zero to shift.

As for how to check/verify in a case like this, using a collimator-type bore sighter is the best way. https://www.savageshooters.com/conte...l-Bore-Sighter

I also had a Leupold FX-III 6x42 that had bad mechanicals (turrets). Adjustments when trying to zero were very erratic and inconsistent which made things extremely frustrating. Sometimes an adjustment would move 2-3x further than it should have, other times could dial in 8-10 clicks and it would barely move the POI at all. Sent it back to Leupold and they got it sorted out, but they never said what the actual issue was that caused it.