So my buddy bought 4 e8s (d2s) a while back and I installed them under his seat in a pretty cool stealthy setup. He called me the other day and said one wasn't working. I looked the sub over after pulling it and this is what it looks like:
There is nearly a centimeter gap where the slugs, including the bottom plate, have slidden away from the top plate, pinching the coil. The coil will not move at all. I've never heard of such a thing, and have no explanation for how it could have happened. My guess is too thick of glue? Anyway he knows nothing about the subject, so I though I'd ask for him, with the added utility of a picture vs. a phone call.
The sub was not abused, seeing less than 300rms in a sealed enclosure off of a Memphis 1000D (4subs). The coil does not smell burnt, there is definitely no over-excursion happening. They sounded great up until this.