On very old, vintage cleaners - the wheels go flat from sitting on them in one spot so long. Also, the horse hair bristles will decay and fall out of the brush bar. Interesting that in very old cleaners I receive for the Museum, ones that have sat MORE than 60 years without being used, the dirt in the bags degenerates to the same point. You cannot see carpet fibers or threads any longer, just hair and beige fine dust. And of course, rocks and pennies. The bacteria picked up with the dirt eventually rots all of the bag's contents - and you're left with this beige fine dirt. After the 100th old vac bag you clean out, you start to notice a pattern.