I've had a number of mispicks from MB, but no more frequently than any other outfit and MB has always been very good about correcting their mistakes. A couple times they've mistakenly sent me an extra bag of grain or something I didn't order, I always inform them when this happens cuz I don't want to mess up my beer karma, they always say just keep it.
As someone else previously stated, MB in my mind is a great model of what an online retailer should be - which makes me worry that they will be purchased by some big outfit and systematically ruined.