Human moment? Or disbelief? Could it be that +Peter's faith failed, as ours does? But, unlike us, +Peter has seen Him, ate with Him, and witnessed all His miracles. I think that's a bit different than our experience.
Human moment. Peter's faith didnt' fail because it wasn't His own. It was revealed to Him (Matthew 16:17). After the resurrection, the Angel says go tell the disciples and Peter... as a way of saying to Peter, even though you failed, You are still mine. Nothing has changed in that regard. Now get up and lets go forward. That's grace. The world would have said "Peter, you blew it." God said "Peter, yes you failed, but I overcame your failure and You belong to me."