collision course
 

Dead ahead, through the pitch-black night, the captain sees a light on a collision course with his ship.
He sends a signal: "Change your course ten degrees east."
"Change yours ten degrees west," comes the reply.
The captain responds, "I'm a United States Navy captain! Change your course, sir!"
"I'm a seaman second class," the next message reads. "Change your course, sir."
The captain is furious. "I'm a battleship! I'm not changing course!"
"I'm a lighthouse. Your call."