Rubber-backed rugs can sort of disgustingly fuse the rubbery bits to hardwood floors over a few years. I reckon I've had this rug for about two and a half years and not moved it. Unsurprisingly, it's worst where I put weight on it more often.
after-and-before images here: (since I uploaded them accidentally out of order)
It took about forty-five minutes of scrubbing with goo-gone to get rid of all this shit.