I had a kind of similar situation happen once at FNM. I had maybe gone to FNMs 3 or 4 times before, still new to the environment noobish still. It was the first one after Amonket dropped and I wanted to try out a mono black brew. 4th round (2-1) I plop down at the table, greet my opponent and start shuffling.

Exchanging some pleasantries, he asks me what I was running and I just tell him "Some mono black, cryptbreakers and what not". To which he replied gleefully "Oh yeah? Dude me too, I run that too--mirror match time".

So game begins. I play a land and pass. He plays forest and passes. I pause, and I think to myself wait what? Maybe he's got some weird tech?? Nope. Turn 2 snake, he's actually playing B/G counters. Realizing this and having a great hand, I proceed to kill every single thing he puts into play. Eventually I got an Archfiend of Ifnir out attacked to face twice, resolve a second and he just scoops.

Second game was pretty much similar to the first but with zombie flood, he scoops again. We sign the slip in silence, and I get up and leave.

All in all I laughed at that early exchange, and told myself it's probably a better idea to just keep quiet about your own deck.