You, James, Sirius, Remus and Peter were running away from McGonagall who you made the joke on. You hid behind one of the walls. When McGonagall seemed to give up catching the marauders and wanted to leave, she sneezed.

"Bless you!" James said cheerfully.

You grabbed your head and hissed, "James, you fucking idiot..."

Remus made a facepalm,

"Why do you have to be yourself?" Sirius covered his face with his hands.