Croatian and Serbian are basically the same language. That's what everyone says. Serbo-Croatian was the Yugoslav standard language and since Yugoslavia broke up some differences have emerged, mainly just for Serbs and Croats to be able to separate themselves from the despised other.
One such word that is completely different is tomato! Paradajz in Serbian and rajčica in Croatian. However there is a connection, my Serbian teacher thinks, between these and that is paradise. Raj in Serbian and Croatian means paradise and čica or something similar is added to the end of a word to make it cuter or smaller. And paradajz phonetically resembles paradise so even though these words in their language bare no similarities, it seems possible that the origin is the same for both. And I love tomatoes so they are a produce of paradise in my opinion as well!