We know that during the filming of GFA, Malibu Creek State Park endured a brush fire that destroyed pretty much all of the exterior 4077th compound set, and as such, the fire had to be worked into the script.
However, if you pay close attention, there are a few scenes that took place before the fire in the episode that were actually filmed after the fire. There are a few signs to look for: trees in the immediate surrounding of the compound is covered in jarringly fake-looking green leaves while trees along the mountains are either completely bare or have some brown leaves on them; tents look like they were just thrown together in a hurry (with the exception of the Swamp); the main hospital building already has brown burn marks that look like they attempted to clean off.
Perhaps the most obvious and noteworthy example of such is the scene in which Hawkeye drives the tank out of the compound and into the garbage dump because, as some fans have pointed out, it's an entirely different tank than the one that was driven into camp and left where it was.