this is a pretty big known problem
it involves the science icons - if you add 1 tech, you cannot have any tech use the last icon, which happens to be Writing. the next are ones are warrior code, theory of gravity, etc. on up.
and you cant just add new icons to help; wont work (tried it)
the only way around this is to just change any affected techs to different icons
also, one thing that annoys me, is that when the sci dude comes up saying completed research, the icon will be all screwed (it will be off by 1 icon for every new tech added) - now this is just an aesthetic error, doesnt hurt the game any, but considering civ3s graphics are the best feature about it, i find it very annoying.
so cuz of this crap im gonna redo the tech tree without adding to it. it kind of sucks, but thats how it goes, unless the next patch fixes this issue - please yes, please please please please!!!! :crazyeyes