I once, for whatever reason, removed and reinstalled the screw from the zoom adjuster on a Simmons scope. Went out in a few days and at 100 yards and wasn't hitting a whole piece of posterboard. The rifle always shot well under 1 MOA. WTF? Went to 50 yards and still couldn't hit it. Tried adjusting the elevation on the scope to no avail. Went to about 25 yards and still couldn't hit it. Then I noticed mud splattered on the board. The elevation dropped all the way and I was shooting into a mud puddle below the target. Went home, took it apart and found the problem. The scope still works ok. Gee I felt silly!