The single worst business decision I ever made was migrating a site on a .net extension over to the .com once the company bought it without making damn sure not to lose rankings in Google. I had heard that permanent redirects preserved pagerank/googleness, but dropped from top 5 on some important terms to outside the front page. This single poor decision/poor execution cost me and the company tens of thousands of dollars, which is orders of magnitude higher than the additional credibility the .com would've brought.

I suppose I had to learn that lesson sooner or later, so I'll go glass-half-full and be grateful that it was on tens of thousands early in life, instead of more later. But that was a pretty expensive lesson.