Especially from Comcast. And if you return equipment, be sure to get receipt.(Not that that always works either.) BTW, Comcast has the second worst customer service in the US.
Here's a recent article about an extreme case, but if you go to the bottom of the piece, read the comment section.
My own latest case was pretty simple. After renting a Comcast modem, I bought my own, and brought back the rental even though I had a receipt for the return. Since DW pays the bills, I don't check them.. after six months, I found a bill where I was still being being charged for the modem. Fortunately I had kept the receipt for the return. On showing the receipt to the customer service clerk, she agreed to take any "future" charges off my account, but I was responsible for the six months rental. A $50 lesson.
Somewhere in the 40 page comcast agreement (the size of a novella), there is an obscure clause that failure to notify them of any overcharges immediately, absolves them of responsibility and they can keep the money paid... even with proof of non-delivery of goods or services.
That wasn't the worst one though... four years ago, upon leaving Florida and cancelling my Comcast account by turning in my modem, (they did cancel the service), they apparently sent a separate bill to my closed mailbox, for the maximum amount of all available equipment $400 that they offer. I only had a modem. $60 at the time.
Since we didn't get the letter(s) and they didn't get the $400, after three months they turned my account over for collection... where the collection agency and the credit rating services found me. It took a letter to the state attorney general to clear this up.
Keeping receipts is a good thing to do for all sources, ... but ESPECIALLY for Comcast.
BTW... If you don't think this involves you, watch for the OK by the FCC, that will allow Comcast to become a monopoly.
Where we live there are NO other choices except dialup.