I'd just moved into my first apartment and had bought my own modem. Tech comes, sets up the internet, the internet doesn't work. Tech blames my modem. My response is something to the effect of "give me a break", but ultimately do order a new modem from Amazon, if only so I can show the tech that two different modems both don't work.
Turns out, the first modem was defective. The second one worked fine and my internet has been great. I felt kind of crappy for arguing with the tech, when it absolutely was my hardware at fault.
Recently they updated my area to DOCSIS 3 (or 3.1, not sure). My DOCSIS 3.1 Motorola modem (found this out later) has a firmware bug that causes random modem lockups. Comcast went as far as replacing the cable from the telephone pole to my house in the middle of winter (it was ridiculously cold that day), running signal analyzers, and various other things before looking into the modem.
Also, Motorola Modem customer service was shockingly good as well. They sent me a free replacement modem while they worked on the firmware updates. Provisioning that temp modem was as simple as using the online provisioning.
Now, it would be nice if I didn't have to set annual calendar reminders to call them for when they increase my monthly price by 40%...