Never had that problem. I pay a fixed rate every month, so there isn't much for them to screw up except changing the rate without my knowledge.
We had to cancel Verzion fios as they messed the bill up about 40% of the time. Even after we canceled they still messed up the final one as well.
So far Cox has been good. Did a promotion this month on their internet so will see if they get that right.
Never had a problem with FIOS in fact when I called to ask about a faster Internet plan they gave it to us for the same price we were already paying. It costs less for faster speeds compared to Comcast in my area. And FIOS doesn't cap you at 250gb a month, which I couldn't easily deal with as I already hit 300gb+ easily.
You'd think that a fixed rate would be easy...but Uverse used to fuck it up EVERY month...
AT&T couldn't get our bill right one month in a row...Every month, we had to call and complain...we'd get a credit...that wouldn't show the next month...so we'd call and bitch...they'd adjust the bill and issue a credit for the trouble...that wouldn't show the following month...so we'd have to call again.........That went on for almost a year. We dropped them and went to Comcrap. We had ONE billing issue...when they changed our internet speed from 16 to 30 Mb/s...they raised our rates by about $2/month without explaining. Once we got that explained...I was fine with the increase...and the bill was right every month after that.