He made a stupid mistake. I have a few issues with calling Fedor the best fighter to date:
1. The Heavyweight division isn't as good as the LHW and the MW divisions. Compare the skill sets of Shogun, Machida, and A. Silva to those of Fedor, Lesnar, and S. Carwin. I'd argue that - despite the vast weight difference (with Lesnar and Carwin) - Shogun would have an excellent chance at beating any of the top-3 HW fighters. This is highlighted by R. Couture's success in the HW division (speaking of his most recent holding of the belt), when it's clear that he wouldn't have a good shot at any of the top LHW fighters.
2. It's not clear that Fedor would beat either Lesnar or Carwin. Would like to see him fight the winner of that fight.
3. Fedor could easily get down to LHW, and I don't think he'd beat Shogun, and it seems 50/50 (at best) that he'd beat Machida.
That said, Fedor's resume is impeccable, and one mistake doesn't trash that. If Fedor removed his head from his *** and fought some key fights, he could easily regain - in my eyes - the status of best fighter of all time.