I have had it happen with a few scopes over my years, I don't feel they are junk, they just are not scopes I would choose to take on a trip where you range an animal and whip the knop to elevate..... Once locked on they are good to go, but the slight tap mentioned above needs to be just that, a friend I was shooting with 2 years ago had the plastic adj knobs both cracked on his scope (just the part you grab with your finger once the caps are off) I bet it is from the not so gentle tap while sighting in.
Hope you all have a fun, safe hunt this fall. - thejanitor