It's impossible to save game during combat, but the game often tries to autosave anyway and fails, which leads to some buggy behavior. The "saving..." message doesn't disappear, as if the game couldn't complete the autosave, and if you attempt to /save after that, the game freezes.
This bug happens fairly often during a bandit encounter, and also happened to me once during a battle with a legendary monster. It gets quite annoying, as I cannot save game after killing the bandits and grabbing their loot.