I am an Idiot

"Idiot"?

I know better.. Anything can happen after sighting in that could throw it off that much.

870, I don't know which make and model you have, but I found the scope mounts to be very critical and susceptible to bumps on my Marlin .22 levergun.

Loved the accuracy with some big glass on it, and my old eyes needed the help, but that short distance between mounting screws on the receiver made it very "tender". I developed the habit of carrying it only in the case, never just laying on the seat or back floor of the Jeep as I had before adding the scope, and repeatedly removed the mounts and looked for the cause of the problem and retightened everything, to no permanent avail. It would seem good and solid and repeat nice tight, accurate groups, then some unnoticed bump would knock the scope out of alignment. Even changed scopes a couple times, thinking it was something internal in it, but the same scope would hold zero forever on my Ruger semi-auto.

I finally gave up on big glass on the Marlin and went back to open sights with fiber optic front and rear , and use it more during later squirrel season when they're more likely to be on the ground or moving in more open foliage, taking the Ruger with the big scope for early season.

Had one other aggravating experience with scope movement on a Winchester .22 auto I bought new back in the late 60's. It would drive tacks during a sighting session, as long as I was just shooting and laying the gun down on its side between target changes, but anytime I reloaded or took it home and returned to shoot another day, it was pretty much a toss-up whether the zero wiould be in the same spot. Again, repeated removal and inspections and finding no apparent problems had me stumped. Finally, I was sitting on the couch contemplating either a new scope or just going back to open sights on the gun as I held it between my knees, butt on the floor. I absently rested my hand on the muzzle while watching something on the TV news, and it took me a few seconds to realize that I was actually feeling random barrel movement at the muzzle, with no movement at the receiver! Checked it out, and sure enough, the barrel wasn't held snugly. The movement was slight enough to not be noticeable in normal handling, and the looseness was allowing it to repeat zero very well when laid gently aside in the same position every time, but made it a complete guess as to whether it would be on after other movement, no matter how gentle. Only showed up with the scope on, because the iron sights were mounted directly on the barrel.

My B-I-L had a similar situation with a Remington Nylon 66. Great little gun and a tack driver for an auto while he was using the iron sights, aggravating POS when trying to use a scope. Finally saw that the problem was the scope mounted to the stamped steel cover on the receiver that was held to the actual receiver frame by a couple of loose-fitting rivets.

Just a couple things I thought you might want to check in case you have any continuing problem with losing zero.