Waterfall_13
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Or another example, this time related to hero skills. I attack Temple units, kill angels, their death animation plays, and then, without any animation, they are resurrected and give me Jigguards. Why? Even opening the unit window doesn't provide any information on this. It turns out it's some kind of faction skill of the Temple, which resurrects units after death! You can only find this out through trial and error, or by asking people who know the game well.
Well, your frustration is understandable; I was also surprised by what happened in the final stages against the AI, when a killed enemy suddenly came back to life and blocked my archers. (In essence, this is a hero specialization mechanic from Heroes 5, where units created ghosts after death).
But I think this isn't the kind of "issue" you can blame the game for. It's part of the game's learning curve (an exploratory experience). After all, all games have their own quirks and hidden aspects that you won't understand until you stumble upon them. In your case, you encountered an unknown mechanic that is described in the game; you just hadn't come across it at that moment, and now you know about this mechanic and won't be caught off guard again, just like me =)