Some of the biggest Business blunders in history...

[Freudian]

Facit was a company that made great mechanical calculators. They kinda missed the train when it came to the electronic calculator but continued to make mechanical ones until their inevitable demise.

Levi Strauss is another company that has gone from world leader to struggling today. I don't know if it depends on a single huge mistake. I think it is mostly not understanding the changing market and being so content with being number one that they stopped trying hard enough.