Fehldiagnosen !! ??

Two medical students were walking along the street when they saw an old man walking with his legs spread apart.
One of the students said to his friend: "I'm sure he has Petry Syndrome. Those people walk just like that."
The other student says: "No, I don't think so. The old man surely has Zovitzki Syndrome. He walks just as we learned in class."
Since they couldn't agree they decided to ask the old man. They approached him and one of the students said to him: "We're medical students and couldn't help but notice the way you walk, but we couldn't agree on the syndrome you might have. Could you tell us what it is?"
The old man said: "I'll tell you, but first you must tell me what you think."
One of the students said: "I think it's Petry Syndrome."
The old man said: "You thought.......... but you are wrong."
Then the other student said: "I think you have Zovitzki Syndrome."
The old man said: "You thought.......... but you are wrong.
So they asked him: "Well, what do you have?"
The old man said: "I thought it was GAS........... but I was wrong