For me it's:
#5. Sony setting a ridiculous launch price for their PS3. This gives the public a bad first impression of the console, indirectly benefitting the Wii's image as the "cool console to have".
#4. Sega quickly releasing many peripherals for their thriving Sega Genesis (32X, Sega CD) and dropping support for them as quickly. This alienated both gamers and developers and cost Sega loads in cash.
#3. Sega designing the Game Gear so that it requires a whopping 6 AA batteries to function, and that it drains the battery life extremely fast. Most consumers who haven't heard of the rechargeable power pack are hence discouraged from buying the system due to the high long term costs of playing the handheld.
#2. Nintendo insisting on using cartridges on the Nintendo 64. The higher costs and limited space make many games releasable only on the PSX (without significant compression, that is). This alienated third party developers and harms Nintendo's public image.
#1. Allocating only 6 weeks of development time to the E.T. game on the Atari 2600. The lack of time and thought that went into that game led to most of the public perceiving the game as rubbish and indirectly caused a video game crash.
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