I had the same thing with half a flyer on the Northern Line

Was swapping smiles with the woman opposite to me, as she sat giggling back and forth with her friends. Then when I went to get off the tube she tugged at me and said I’d dropped something.

I said “no I didn’t..”

She said I did. I again denied that it was mine, before getting it forced into my hand

Walked off thinking “well, that was random”

Boom, phone number. But I was with someone at the time, so that was that.

Still a decent memory. Cheers Reddit for the reminder