OK... my first definite entries to the list:
Sealife DC800 Maxx kit. I would have been perfectly happy with it if the stuff actually worked for any length of time. In the 10 months that I have owned it, I have had the DC800 camera replaced 3 times for lens defects (they switched out a DC1000 for the 800 when it first came out), I have had four trips to the factory for defective flashes, and have had to have the DC1000 case repaired for a shutter button that sticks if you shoot a picture deeper than 50 feet.
I don't regret my ScubaPro Glide Plus because I still use it in the pool and will be using it if/when I do the DM training stuff in the pool... but I have since replaced it in my diving stable with a BP/W.
I regret buying yum-yum yellow fins when I am swimming in the ocean where they look stupid, but I don't regret them when I am swimming in a quarry with less than 5 foot visibility.
I regretted buying an Suunto SK7 until I replaced the horrible wrist boot with a bungie mount.
That's all I can think of for now.