This isn't for doing evil, I swear.
Just a quick unbuffered JTAG for my Motorola Surfboard 5101 cable modem. It was $10 used and needed a flash back to non-hacker firmware for use on Comcast HSI. I got the JTAG pinout from a cable modem hacking forum, wired it up, and connected it to my account with no issues.
I left the JTAG attached afterward, leading to some weird looks from Comcast techs when they came to hook me up.