That happened a while back when I was in the hunted server. The guy looked like a engineer but he said he was a spy! We even have him switch weapons and you could see the engineer holding a revolver, and shooting it. xD
I had worse. I was in Warpath chasing what to me looked like a friendly engineer with his shotgun out so I could help him possibly build something, and I found out (the hard way) that it was actually an enemy Pyro. I freaked out when I saw fire shoot from it's shotgun.