I've had several go bad, but haven't always sent them in. From memory, the list includes:
*Several older K4s
*A Burris 4x
*Several mid-priced variables (can't remember all the brands, but most were purchased used on rifles)
*Older Luepold 4x (need to send that one in now. wont stay within 20 feet of zero)
*Leupold 2-7x VXII (Went bad on a light .375 and after Leupold repaired that scope tracks better than anything I own is back on the .375)

Most of my scopes are Leupolds.