Wanted to post here in case somebody encounters a similar issue. 

I have an HK VP9 pistol and a Holosun Green Dot. The pair works beautifully; I can keyhole shots all day. But then, about once every two months, I go to the range, and the green dot is off by a good 2-3 inches on a 10-yard target. It happened twice already, which is concerning. I want to trust my weapon in case of emergency. 

At first I thought I messed with Holosun adjustments when cleaning the gun, or they shook loose from all the shooting. Wasn't the case. Then I thought the Holosun itself had come loose from all the shooting, but the screws were tight; I always check as a part of maintenance. 

Turns out, it's funnier. As a new gun owner, I, of course, overoiled the gun when cleaning it, to the point that oil was leaking from every pore. I'm still wiping it every time I go, and the thing just won't quit leaking oil a little bit. 

It's not much of a problem, except the mounting holes on the VP9 were drilled all the way through the slide (see the picture). So the oil, leaking from every pore, made its way up the screws and under the Holosun, creating a nice oil lake under it. That turned what was supposed to be a tightly mounted Holosun into a boat. Every 700-800 rounds, that boat would depart the pier just enough to mess with my aim. 

Big thanks to a local range employee for troubleshooting with me and figuring out the issue. I'll carefully wipe all the internals from excess oil (I'm sure it will still leak from somewhere) and seal the screws with Loctite.

 

Greed Dot Random Misalignments