There's this video, but it's in Portuguese:
He says he was doing a class for combat veterans in Croatia when the dude that was a famous soldier for killing many people in the war came in. Werdum chose him to roll with (after already rolling with many other veterans) and the dude caught him in a guillotine and pulled guard. Said he was very strong so Werdum was already seeing stars, close to passing out, when he finally managed to pop his head back out.
Than he said the guy wanted to leave, since he already had almost tapped Werdum, but Werdum kept him in there, asked the translator to say some things (to give him time to catch his breath back after the guillotine) and proceeded to tap the guy.
He also said he has no problem tapping in practice but in that case it would be embarassing as he was Crocop's jiu jitsu coach at that time.