Anyone here have horror stories to share about out-of-spec parts encountered while trying to build up an AR?

So far, my worst (and current!) problem is a bolt stop that's actually bent towards the button end.  It'll almost fully seat in the slot, but the slight bend keeps it from seating properly.  If you force it into place, it'll go, but then it binds tight and won't move freely.  I've notified the vendor and hopefully they'll send me a replacement part soon.

I had a problem with my J&T lower kit bolt catch.  Mine had a little dimple in it.  It wasnt much of a horror story becasue i was able to fix it by sanding down both sides of the bolt catch,  Other than that everything took less than thirty minutes to put together.