Right before the game winning penalty kick in the England-France match last sunday Zinedine Zidane vomited twice while on the pitch. He then stood up and scored the goal required and expected of him.
Here's the video (with Swedish commentary).
Physical exhaustion and dehydration possibly play a role as well but one would guess that the main reason for this incident is quite simply the enormous pressure on a player in that situation. That pressure was certainly augmented when England's star Beckham failed the same test of cool a little earlier in the second half.