Luckily I haven't done it with rifles yet. But scopes many a time!
I purchased a Nikko gameking 6-24x50 IR AO, huge thing, that sat on top of an AA S200. Shite combo, too many bells and whistles not enough good glass.
I've let go of a few great scopes and thought 'why did I do that???'
Simmons etc 1.5-5x20, Bushnell trophy 3-9x40, and recently an mtc genesis UL 3-9x40. All great scopes. Luckily I managed to hold onto another Bushnell trophy! Moral of the story don't let go of good glass