OMG I’ve had that with someone before! I was out walking and walking towards me was a guy who looked like the brother of the guy I’d just recently (at the time) broken up with. Having no beef with the bro I thought I’d be civil and say hello. We were both wearing sunglasses and he enthusiastically said hello as well, then we both took off our sunglasses and discovered that neither of us were who the other thought we were lol. We both just couldn’t believe it, it was so weird and random.