I'm not sure if it deserves its own topic, but here it goes. If you're overwriting an old type (item, building, spell, Abilitybonuses, raceconfigs), then some variables will be saved from the old version. So if you just copy say a building to a mod file, it will require twice the amount of resources to construct.
I think I've now found out that only <GameModifier> values get doubled - everything else gets overwritten. So you should be able to copy everything from a type except the gamemodifiers and not get a doubling problem.
This also presents another problem. Since the original gamemodifiers cannot be overwritten, you have to use the original gamemodifier as a startingpoint and make your own gamemodifier a "modified" version of that. For example, if you have a building that costs 100 gildar, and you want to reduce that to cost 75 gildar, the correct modifier would actually be
instead of the logical <Value>-75</Value>
Comments? Anyone found any other tag that can't be overwritten?