Welcome kit fail
I suppose this is more of an FYI than anything, but I figured it was funny enough that I should let someone over at Comcast know.
I got a thank you for choosing Comcast email, with a "custom video" just for me which ended with the instructions to click on a link to register for an account, and another to confirm my installation appointment. When I click to register however it fails to recognize my account number which was provided in the same email as the video link, and when I attempt to at least confirm my appointment it fails with a totally generic error message.
FWIW: Yellow boxes are for warnings, red boxes are for failures.
Nice generic error page when attempting to confirm the appointment.