Shortly after coming on as a second-half sub in Argentina's last World Cup warm-up, Lionel Messi doubled over and appeared to vomit on the pitch.
He's done it at least a half-dozen times with Argentina and club team Barcelona, mystifying doctors and fans alike.
"Nerves," says Argentina coach Alejandro Sabella, and that's a diagnosis as good as any.
The Argentina captain and four-time world player of the year is under tremendous pressure to lead the Albiceleste to its first World Cup trophy since 1986.
"I reckon that in these moments there is anxiety more than anything," Sabella said before the team departed for Brazil, where it opens its World Cup campaign against Bosnia-Herzegovina on Sunday. "It's difficult to remain calm."
Top athletes throwing up before, during or after a competition is not unusual. Sometimes it's because of the physical stress on their bodies, sometimes because of performance anxiety.