Obviously, in modding this game, every modder is going to come up against some. probably many bugs. This thread is meant to solve some of the problems that you've had. If you have any questions, or answers (I have both :D) about specific bugs, please post. (Make sure it's related to your mod, not just the game itself- that'll go in the "tech support" forum) I'll start out:

1) Problem: pops up: "Tag: WHATEVER in Info Class was incorrect. Current XML file is: somefilename.xml"

Answer:
Probably the first bug that many of us encounter. It means that there is a reference to some unit/civic/whatever that doesn't exist. It can happen if you remove a unique unit from the Units/Civ4UnitInfos.xml file, but not the reference from CIV4CivilizationInfos.xml defining the Civ's UU. Fix by removing ALL references for tag WHATEVER in ALL files

2) Problem: pops up: "Tag: in Info Class was incorrect. Current XML file is: somefilename.xml"

Answer:
If instead of WHATEVER tag, it's blank, it means that you closed a tag (made it <TAG/>) that's not supposed to be closed. To fix, you have to find what you messed up in somefilename.xml (not easy sometimes :P) and either give it a value (can usually be NONE)

3) Problem: game crashes to desktop when I found a city

Answer:
In my particular case, I had to move my mod's folder from the My Games/Civilization 4/mods folder to the Fireaxis Games/Civilization 4/mods folder...apparently this is something you have to do as your mod gets bigger :P

When I was making a Wonder in SotM and I had it's ART_DEFINE tag looking for something that didn't exist I crashed everytime when it was completed. My guess is you're founding of a city is referencing something that isn't correct, for some reason.

maybe it's in the civilizationinfos.xml. that refers to the building style of the citys (though it's somewhere in the middle of a civilzation code, so if this isn't changed, it's not very likely that this is the problem)