Not really sure about this. For a long time, I didn't have enough money to make bad impulse buys when it came to games (for a long time I didn't have money for games at all). By the time I was buying games, it was almost always possible to rent or borrow first.
During the Wii lifecycle I bought a bunch of pretty bad games out of curiosity - but it's hard to call them "worst" buys because the pricing was extremely low, matching my expectations.
So in the end, my worst impluse buy related to gaming is probably grabbing a Wii U as soon as the basic sets dropped to $200. The Wii U console sat unplayed for many months because I was still enjoying Wii, and because wireless simply wouldn't work with my router. It gets more use now and trading routers fixed the connectivity issues... but my idea that "things will get better soon and prices won't go any lower" ended up being (mostly) wrong. That said, I'm still pretty happy with the purchase - it just that the timing/value wasn't as good as I had hoped.