I was on the the train once, bored as hell (because trains in Romania are really slow) and at one point I hear 2 guys behind me, probably as bored as I was, talking about puzzles.
And I thought:
"Oh! I like puzzles. I should join them. I'm sure they won't mind. But wait...what if I have no idea what's it about and I look like a fool? I better eavesdrop a bit and but in only if I know something".
But a window was open and the noise from outside covered part of their conversation.
All I could understand was this:
Guy1: Look at this (mmmmm). It has numbers and letters.
Guy2: it has to be a crossword.
Guy1: That's not possible because (mmmm). But they can be the moves for a Rubik cube.
Guy2: Pff...you're an (mmmm). There are only 6 letters in the Rubik cube moves. Maybe it's a matrix of something. Letters on (mmmm), numbers on (mmmm).
Guy1: Or a cipher? Man, I hate ciphers. Or a pattern?
Guy2: There are (mmmm), so this is probably not a rebus.
I this point I realized I have no idea what's this about and decided to shut up. Also, I'm a shy guy and I didn't ask them what's this about.
But I am still puzzled (pun intended). What was that about?
In the light of the 2 answers I feel obligated to mention that the fact that letters and numbers are involved is not important.
And I remembered one of the guys saying at one point that they were trying to solve a puzzle from puzzling.stackexchange.com.
Note: (mmmm) marks the parts I couldn't hear, but they are not really important. You can solve this without knowing what I missed. They are there only for dramatic effect.
Note 2: (not part of the puzzle, just "fun" fact): The trains in Romania are really, really slow.