Completely unsurprising. Comcast billed me for imaginary hardware, twice.
I set up cable Internet service with Comcast at a vacation home with no TVs in it about two years ago, just to be able to surf the web, etc., while there. Sometime around December, 2013, Comcast apparently decided that we needed TV service and shipped a set top box to the address, where it apparently sat, covered with a light dusting of snow for months (it's a vacation home). And, of course, they billed us for TV service and a rental fee for the box for months. I got that straightened out, and a credit issued.
A couple of weeks ago, I looked at my bill from them, and, lo and behold, they have been charging me an $8.00/month modem rental fee. I bought my cable modem from Fry's for less than the $96/year that would have ended up costing me.
As far as I can tell, when they removed my non-existent TV service and took back their set top box for the imaginary TVs, they stuck on a modem lease fee.
I have finally gotten that straightened out, and, no doubt, will have some new billing failure from them in a few months for hardware or services I did not request, own or otherwise purchase from them.