I hate to use NANOG for this, but support has now ended a chat with me twice without fixing anything, they just kicked me off.
I'm not getting an IPv6 address on the Comcast provided cable modem/router. I'm not getting a PD. My machines thus have no IPv6. I've hard reset my router 4 times while working with Comcast, and I've been told to do things like switch to a static IPv4 address, which shows a level of clue that is scary. And before that they were convinced it was a wireless problem even though I have a wired connection, and told them that multiple times. I've wasted two hours with Comcast today, and even when I asked for escalation I got nothing. Just hung up on. It's honestly the worst customer support I've ever received. I don't think I ever got them to understand the difference between IPv4 and IPv6.
I experienced a loss of Comcast IPv6 service at my home I think it was last Monday... My pfSense reported loss of reachability to the IPv6 gateway, IPv4 still worked but I had a period of intermittent high latency... Does anyone know if Comcast is monkeying around with their IPv6 network?
It's starting to become more typical.
I finally resolved an issue after two weeks of fighting with them.
A remote office could send traffic out, but couldn't receive traffic.
I ran tcpdumps on the firewall, and did everything to convince them it
wasn't my problem.
They still insisted on sending out a 'tech' to check the issue. When
the tech hooked up his Windows XP laptop to the modem and was able to
pull up Bing, he said everything was working fine. We were told we
would be billed for the tech coming out.
The last time I called in, I *finally* got someone who was studying
for their CCNA, described everything, and he spent about an hour on
the phone troubleshooting. Finally he re-flashed the modem, reloaded
the config, and manually configured the static IPs on the modem.
Everything immediately came up.
http://xkcd.com/806/
Maybe Comcast train the level 1 techs that if someone says "NANOG" you
get transferred to someone who knows routing... 
-A
It's starting to become more typical.
I finally resolved an issue after two weeks of fighting with them.
A remote office could send traffic out, but couldn't receive traffic.
.....
http://xkcd.com/806/
Cute.
Maybe Comcast train the level 1 techs that if someone says "NANOG" you
get transferred to someone who knows routing...
And Charter gets you the business NOC if you call level1 tech between 2
and 6AM eastern. Unfortunately this is the fiber-service noc so they
can't do much with cable nodes. At least they know what a router is,
and ping and traceroute.
Reminds me of a call I made to the local power company some years back;
the transformer for my end of the block was rather undersized for the
more-recently installed customer air conditioners, and my line voltage
was around 85 in the afternoon. Computers don't like that 
Called
the trouble line, said my voltage was low. She asked how I was
measuring it, I said the magic word "Fluke". She then said it would get
reported right away. Of course, fixing it was a major undertaking (had
to replace the 2400 pole-top lines with 16kv, and add a transformer), so
it still took a year for them to actually fix it (and a day without
power while running the new lines)...
At least that magic word was in the script...
-- Pete
I had a similar thing with Shentel.
When I finally started sending them screenshots of Wireshark ARP traffic I got to talk to someone that knew something. Turned out another customer was advertising they owned part of my IP addresses.
Gary