Don't ask that question. Hahaha. I kid a bit but you're asking for horror stories, and let me tell you they get gruesome. I've done the trade and pray with Samsung and had no issues. The process is kind of typical I guess. They mail you a device with a shipping label for your old device. You get 14 days before they start sending you email reminders warning you that time is almost up, and then offer you a few days more before they charge you full price for the device. I think they say 14 days but actually give you 21, don't hold me to that but this was my experience for a device I thought I had mailed and hadn't. In the meantime, when you're done transferring everything over and hoping that you haven't forgotten anything vital, they want you to factory reset the phone from the settings menu so that all of your information is removed and the phone is ready to be set-up. They will automatically reject your trade if it isn't reset correctly or if it isn't reset at all. You pack it up, slap the label on it, take it to your nearest Khol's, send it, and pray it arrives at Samsung and pray that they accept it. This goes back to the horror stories I mentioned. Every year AC members report devices they say Samsung wrongly rejected. "My phone was perfect and they're saying it has damage.", "I tracked the package to a FedEx location, usually in Texas, and it just stopped and never left.", "They say they never received my phone and now they want me to pay full price, but I mailed it.", are same basic stories that pop-up every year. I've done it 4 maybe 5 times and have not had an issue. Knock of wood. Each time I've done the return, I call myself doing things that I'm hoping will protect me if I should be struck by one of those stories, like videoing the phone condition, as I'm putting it into the box, at the FedEx drop location, hopefully in front of witnesses, but in all likelihood it only makes me feel better.
Like I said, I've done it at least 4 times and have had no issues, but I've heard if they reject your device they of course charge you full price, then give you a much lower partial credit for your device, that somehow won't count towards your new phone but can be used at Samsung dot com, because they don't return unsuccessful trade ins. At least I've never heard of or seen anyone get theirs back. Really though I think if you're honest with yourself first and Samsung second about the true condition of the device and trade this way, you'll be fine. Not always but many times I've felt people weren't being honest either with themselves or with Samsung and then suffer from their own poor choices.
A lot of people like to do trade-ins with their local dealer. They can see the hand-off, know immediately if the trade is accepted and at what value, and a lot of those places also help users get set-up the device, if that's something you need or want. BB's insistence that they set up or help you set-up your device is kind of annoying to me. Not sure if they still do that.
Let me know if this answers your question. Just wait for the dark stories to roll in but take them with a grain of salt.