The only ones I can remember right now is in that it's not possible to have a civilization sub-tag in <unit>, and several events can't be contained within <eventjoin>, <eventunion>, etc.
As a side note for planning demos, I won't be able to make big progress on the ai until probably april (it requires lots of focus, and I am a bit busy right now), so I'll mostly fix things until then (and maybe add a military panel).
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