theres plenty of different words that older generations would know what they mean in the other language because theyve been exposed to it but for younger generations that havent its completely alien
if a serb would ask a croat what month it currently is and the croat answer in croatian the serb would stare at him blankly thinking what the fuck is this retard talking about (october in serbian is oktobar, in croatian its listopad)
since croatia has been ruled by habsburgs and serbia by ottomans for centuries, serbian has more middle eastern loan words while croatian has more germans, but croatian nationalist movement sought language purification which came with the "(re)invention" of words, so just like with listopad, a serb would find a croat calling an airplane zrakoplov ridiculously silly
another massive difference is spelling, and i am not talking latin script vs cyrillic script here, i am talking about spelling in the same script, latin
serbian is writen phonetically, croatian isnt
if you asked a croat to write down the largest city in the us he would write down "New York", if you asked a serb he would write down "Njujork", if the communication was in written form and was involving foreign topics, intelligibility would fall down even further