When I purchase a game if it has a good price (full priced or a little sale), and it appears on a big sale (bundled or not), I don't regret it. I never regret buying something good at a good price even though it could be discounted after. In that case, I am just happy for others who can give the game a try. I don't regret it because I was interested in that game, in the first place, after all. And if the game is good, I believe that my "overpriced" purchase (which is not, in my point of view) is some kind of reward for the developers.
If I purchase a game or a book that I can't play or read at the moment, if it worth it at the end, I don't regret that purchase. It isn't a matter of price (if it wasn't really overpriced, obviously). It isn't even a matter of time, if I am sure that someday I will read/play it.
So I consider that I had very few stupid/unneeded purchases. But when it happens, I am some kind of angry or the deception is really huge.
The first time, it was Powermonger on the MegaDrive when I was a child, twenty years ago. I had discovered strategy games with Dune II, and I thought I would love the genre. So when I saw a second-hand box of Powermonger mentioning "real-time strategy", I bought it, hoping it was similar to Dune II. The box hadn't its manual but I really thought I wouldn't need it, having spent many hours on Dune II. But it wasn't the same kind of game, much more similar to Populous. And I wasn't patient enough to try to understand the game without a manual. So I abandoned, and I tried to only purchase games with a proper manual or at least games that I know what to do in, and if critics were good. Yes, I wasn't so naive to trust video game journalists at the time, so if I was interested in a game, I read many reviews to be sure that it would fit to my tastes. And that's why I haven't purchase a lot of games since then, because I was very cautious.
The secund time, it was many years after the first, it was only three years ago. I loved the Total War series, having each game since Shogun Total War (I remember the first screens in video game magazines when EA was the publisher) to Medieval II Total War and its addon. I stopped having interest when I heard of Empire Total War which a buggy mess, and thanks to my cautious attitude, I avoided purchasing it. When Napoleon Total War released, it was just "meh", because I thought that game should have been a addon for Empire and not a real priced game, and the "collector's edition" was full of units DLCs, which was nonsense for me even then. So I skipped that game too. But I bought Alpha Protocol because I was an Obsidian fan, and an old Sega fan too, and I was pleased to know that the original DRM was removed with the latest patch.
And when I first heard about Shogun II Total War and its big collectors edition, I preordered the game. I really wanted to love the game, remembering old Shogun LAN parties. But when the box arrived, I saw that Steam was needed, I was surprised after the no-DRM stance on Alpha Protocol. I wasn't so anti-Steam back then, I just didn't like it. So I installed Steam and saw that even on release I was forced to download many files to "update" my installed game. It was Day-one! And after that I have heard on the Steam store that units DLC were planned. The deception was huge. The only thing I saw from the game was the options screen. I never launched a campaign or a battle. It was just distasteful. So I formatted my harddisk drive, to be sure that no file from Steam will remain on my PC, took the box back on the shelf, back to dust. And it never moved since then. I promised that I won't ever buy a Total War game again. Never again.
And the third time was... the last GOG big sale: Luck of the Irish. The pot "gave" me too many games that I don't want to play because they don't fit my tastes. It was my fault, I was naive to believe that I could "win" after all with games that wouldn't appeal me first but that I should try. So from now, never random purchases ever.
Maybe I should give Powermonger a try since my tastes and skills on strategy games have evolved in 20 years.