It depends what part of the process yor're referring to. IE6's initial development wasn't a mistake. But disbanding the IE specific development team, abandoning all platforms that weren't windows and not updating it for years and years were mistakes and all those things happened in the era of IE6
IE6 is not a mistake. It's old. It first come with Windows XP and Windows XP is 8 years old. At the time, IE6 was fine.