Gerald Ratner, how to ruin a successful high street presence with one sentence.
Nissan Primera, how to totally ruin sales of a popular family car
Commodore , how to milk technology such as the Amiga until its no longer capable of competing
Sony Ericson, taking a successful brand and moving into a new android market and just totally failing at it
3Dfx, ruin their market lead by trying to be bigger then they were and to become a card manufacturer instead of a chip supplier
Nintendo, slipping up and letting Sony take the idea for a CD based Nintendo system and turning this into the Sony Playstation
Sony, Minidisc… Erm well that really was a bad idea
Sony, again with UMD movies. Totally wrong
Honda F1, pulling out when the team had designed a winner in 2009