was it really squishy? are your rubber feet fairly soft or hard rubber? do your rubber feet have a fringe of left over rubber from the machining process?
It really sounds like rubber left over from the feet of your case. Worst case scenario a wire was stripped a little, but I don't think that there is any risk that you stripped the rubber to a point where the wire would be exposed.
The only way to be entirely certain is to open the case and inspect every black wire, probably wouldn't take to much time and would be worth your ease of mind.
I remember it being pretty squishy. My case has two soft rubber feet in the rear that actually leave black marks on my desk when I drag the case across it. Also when I rub my finger across the rubber feet it leaves some black "dust" on my fingers. Do you think that could be it?
I'll open up the case again and check out the wires and report back.