psychophipps
Explorer
The earliest occasion I can recall: back in the 1980s, playing The Fantasy Trip. Our PCs are advancing through a forest or jungle. An enemy patrol shows up. Instead of charging headlong at them, we initially fall back from column into line, so when they charge at us, they end up with relatively ineffective team positioning, compared to the PCs, who then destroy the patrol with minimal injuries. My PC is pleased (yay survival!) and I as a player am pleased (it's fun to play an individual bad-ass, with flashy spells or a huge sword, but it's also fun to play a member of an effective team). The DM, however, isn't happy, because that's not the outcome he expected. So he decides, off the cuff, that we were fighting HALF the patrol, and here's the other half, showing up just now.
It was obvious that he was changing the encounter mid-scene, obvious that he was unhappy with the patrol's results when they charged a spear-line and became shish-kebabs, and obvious that he was, in effect, punishing our PCs for fighting with effective teamwork. I mean, if the DM's goal is "this encounter leaves the party with major wounds", then we might as well have the PCs slash and bash themselves, and get it over with, rather than fight as many waves of "patrol" as it takes to inflict that outcome.
"No battle plan survives contact with the enemy." - Murphy's Laws of Combat
Yeah, I've had this happen a few times as well back when we were in High School. I have learned as I have GMed myself that it pays to be a fan of the characters and to cheer their successes rather than get mad about them.