Deisler was regarded as the outstanding German talent of his generation when Bayern signed him in 2002. He suffered repeated serious knee injuries and, in 2003, spoke publicly about being treated for depression — almost unprecedented in the game at the time.
He retired in January 2007, aged 27, saying he no longer wanted to play football. Bayern have consistently spoken about him with care rather than regret over a fee.
Why it did not work: not a scouting error and not a bad fit. It belongs on this list only because the money never returned football, and it is a reminder that transfer post-mortems are about people.
On the fee
The figure above is a reported fee. Bayern, like most German clubs, do not confirm transfer amounts, so contemporary press reporting is the best available record and add-ons are rarely disclosed. Where a fee is disputed, this site says "reported around" rather than quoting a false precision.