About ten years ago my father decided to buy his younger brother a new scope for his 70th birthday. As instructed, I picked out a mid-priced variable Leupold hunting scope and everybody was happy until my uncle mounted it on his deer rifle, an early Browning BAR .30-06, and stepped into the yard to sight it in.
The first 7 shots each zoomed off in a different direction. Back to Leupold it went and it's been fine ever since. Stuff happens.