They wasted the career of the best running back of all time in Barry Sanders.
I never understood why he didn't demand a trade in his second year, and threaten retirement(and follow through) if his trade demands weren't met. It was obvious by his second year how his career was going to play out. Hell, that Lions run in 92 was a total fluke.
Even from Detroit's side of the fence, a trade made perfect sense. In the late 80s, they could have got ANYBODY for him. Detroit could have either pulled off a Hershel Walker/Cowboys style trade, or went to a SB contender like the 49ers and made an attempt to acquire Young.
Instead, they decide to do nothing and wallow in mediocrity to this very day.