1.-3DO
2.-Blu ray on the PS3
3.-UMD
4.-CELL
5.-RROD
6.-Sony and nintendo not making the SNES CD (well a mistake for nintendo at least XD)
7.-Jaguar
8.-32X and Sega CD
9.-Dreamcast's Giga discs
10.-Grand theft auto series...... j/k

 3rd parties left because of Sony's royalty structure and use of much cheaper to manufacutre CD's.

Nintendo not going along with the CD drive.. that really changed a lot of things. Besides that, it's hard to really point out any others. Virtual Boy was a flop, but it was more of an experiment I believe. Obviously the biggest mistake was how Atari and the gaming industry in general handled things in the past, causing that big crash before Nintendo helped pick it back up.

And I do think Sega handled things poorly, or they would maybe still be around.

 Actually despite comomon belief that is a misconception,dropping the deals with Sony was a good thing (look at Sega CD)

What really killed Nintendo was using carts in the N64 compared to CD roms(even if more risky then due to piracy)

CD rom was much more attactive for developers 

 If N64 went with  CD rom ..PSone most likely never had anything near the success it had.

Still too early to say if Blu-ray was indeed a thorn to the PS3. People also thought DVD playback on the PS2 was a mistake, with quite a brouhaha. People were buying PS2s as a cheap DVD player rather than a gaming machine.

Hasn't that been the case for PS3: With hardware being sold, but not a significant software attach rate to boot?

Is a very different case. When the PS2 appeared, the DVD format was growing and starting to replace VHS's. The costs were reducing and  it was the format of the future and people wanted to use them. The PS2 came to fill a "necessity" on those days and that's why the DVD format helped the PS2.

 Blu ray is not filling any necesity. DVD's are very far from becoming obsolete and Blu ray feels more like a "luxury" than a necessity. For taking advantage of the Blu ray you need a HD TV and a 7.1 home theatre. Although HD-DVD died, Blu rays weren't growth bigger and is not helping the PS3 at all, because of the costs for manufacture them.

If memory serves, it took until 2003 for DVDs to actually become bigger than VHSs. That was 3 years after the PS2 launched, and it had sold quite an impressive amount by that time. While the PS3 won't sell anymore near that amount in the same time frame, it won't stop the growth of Blu-ray.

One thing you have to remember is that DVD was around since 1995, 8 years before it overtook VHS. Blu-ray has many years to grow. Besides, the PS3 is one of the cheapest Blu-ray players on the market, and yet it's still undertracking the PS2 by a large amount. That either means that their sales were almost entirely dependent on the popularity of the format, or, more likely, it depends on which system had more games and was more popular.

I lost my train of thought, so I'll stop there. 

I won't add much new stuff, but in no particular order:

1.) N-Gage: I managed a Gamestop for 2 years during the launch of the N-Gage, and didn't sell a single system or game for it. Not one. In two years. Unless you count the free one we got at our manager's conference, which I promptly traded in for store credit when I got home. Um, ouch. Single worst console launch/system performance of all time.

2.) Virtual Boy: Red.... headaches... make it stop.

3.) 99% of anything related to the N64: I loved the N64, but Nintendo did just about everything wrong, and unfortunately at a time Sony seemed to do everything right. The few gems in the N64 collection pale in comparison to the squandered potential of the system. Cartridges over discs and completely shutting out third parties being two of the most glaringly obvious wrong decisions. (Though it did give us the ever glorious Aidyn Chronicles.)

4.) The SNES addon that became the Playstation: Probably a mistake on Nintendo's part, but who knows how gaming would have evolved if it did happen?

5.) RROD: Only thing that stood between MS and market domination this generation of systems. (Well, at least against the PS3.) Giant blunder, and the repairs after the fact aren't enough to overcome it entirely.

6.) Everything Sega did related to Dreamcast: Great system + terrible marketing = dead hardware manufacturer. Maybe for the best in the long run, since Sega can go back to focusing on games. (Wish they'd get back to being very good at that again. *cough*NewPanzerDragoongame*cough*)

Most of the others in the thread cover the majority of the other notables. Just shows that no console manufacturer is anywhere near flawless.

I'm going to have to disagree with you on that because their marketing was great, with their "9-9-99," and "It's Thinking!" ads.

If anything, the equation should be: Great system + Missing key 3rd party support = Dead hardware manufacturer.