Not sure if this is what you're looking for, but my worst experience involved unboxing Mage Wars: Arena. I had been pretty excited to play it because I was an avid Magic player back in the day, and not needing to worry about overly expensive cards, AND the ability to use cards from your entire deck at once seemed like a really neat idea (plus the spacial grid element). So I opened up the box, looked at the cards/components, everything seemed great. The game comes with two binder-ish books for putting cards in, which you can later pick from to play in the game. The first book looked completely fine. The second book, I noticed had a sort of dark swirl design through it.

At first I thought it was there to make the book look like it had a sort of burnt look, like a warlock's grimoire, but then I noticed it was only on a couple pages towards the middle. I got a wet paper towel just to be sure, and sure enough, the stuff was coming off, a yellowish brown color. I... really hope I'm wrong about this, but the only thing I can think of is the poop of some sort of rodent or something must have gotten smashed in the book in the factory, where it stayed for god knows how long. I cleaned and disinfected the book (and my hands) several times, but needless to say it killed my Mage Wars enthusiasm. I didn't think to take pictures or anything for proof before cleaning it, so I had no way to return it. So, I put the game away, and haven't touched it since.