The Twilight Zone Season 5 Episode 3

Bob: I know I had a mental breakdown. I know I had it in an airplane. I know it looks to you as if the same thing's happening again, but it isn't. I'm sure it isn't. Look, the reason I'm telling you this... isn't just to worry you. You notice I didn't tell you before.
Julia: I want you to tell me.
Bob: I didn't tell you before because I wasn't sure whether it was real or not. But I am sure now. It is real. There's a man out there. Or a... a gremlin, or... whatever. If I described him to you, you'd really think I was gone.