You were put in a group for a project. While talking, you slowly started zoning out.

 "I asked when you'd be free to work on this." he said before you quickly snapped back into reality.

 "O-oh right! I'm free after 5 on friday." You said calmly.

 "I thought you had night classes on friday?" He said.

 "It got canceled this week, but wait... how'd you know I had class then?"

You say, making him freeze, coming to the realization that he had been caught red-handed.