Sure, everyone has bought something at some point or another that they regretted.  Personally, PS1 and Final Fantasy VII, both of which I got when they first came out, are the biggest regrets for system and game, respectively.  The former mostly gathered dust (other than playing Castlevania: Symphony of the Night) while the N64 was in constant use, while the latter literally put me to sleep and is one of the most boring games I've ever played, and probably the most boring one I've ever attempted to actually play through (failing at that due to the extreme boredom of menu-navigating games like FFVII).

This gen closest I've come is my Wii, but I got Twilight Princess, Super Mario Galaxy, and MLB Power Pros out of it. If new Zelda and Mario come out any time soon I wont have any regrets.

I made one big mistake and since then I always did the research before buying.

On the PSOne I bought 007:Tomorrow never dies. I was 13 and it was a lot of money for a Christmas present at the time. I felt like sh*t for asking my parents to get it.

Since then I've oly bought AA/AAA titles :)

NEVER AGAIN.

WIN!

XD

Time Shift

Gears of War

Bioshock

GTAIV(slightly)

I loved RE5, but regret purchasing the Multiplayer DLC.

I am debating on whether I regret buying a DSi instead of a PSP-3000. I already had a DS Lite and the DSi wasn't really much of an upgrade in my opinion. Oh, I might get some crap for this, but I regret buying The Conduit because of all the hype. For me, it really wasn't a good game. I have recently traded that plus 3 other games to get Call of Duty: Modern Warfare 2. That is a game I don't regret buying :).

mini ninjas

left 4 dead 2

Not really, it is mostly a case of not having tmie to play them, and it feeling like a waste, with games like End War and Battlestations Pacific which I have not really got around to playing that much. I mostly regret buying Lost, but I didn't pay anything for it.... Eye of Judgement on the other hand

No remorse on console choice so far.

However was dissapointed by Assassin's creed (1) - as everyone knows too repetitive and I was also dissapointed by the visuals (having played KZ2 before it).

I regret buying Killzone 2 so early on.

There's nothing wrong with the game in particular, but the aiming wasn't to my liking so once I played single player I never really got into multiplayer. Therefore I don't think it was worth the higher price I paid from buying it within a week of launch (as opposed to later on) because essentially all I got was a single player experience.

I also regret buying De Blob. It was pegged as the first good/great non-nintendo game. I don't mind the slightly kiddy artistic style, but the game play is quite repetitive and the things they do to vary it up wasn't too exciting.

Also the last fifa world cup game (2006 on 360) for $10. Wow that game was a hunk of crap, even at $10.

The only current gen console that I haven't had buyers remorse about is the DS Lite, and I own at least 15 games for the thing.