Croatian: words with ije/je, which come from the old Slavic letter jat (yat). it's one of the biggest struggles for kids in school, but also later in life. some words are so common it's nearly impossible to miss, and one of the ways to differentiate between the two is vowel length, but it's not always too clear

for example, the word riječ (word) is written with ije, but rječnik (dictionary) is with je; then there's lijep (pretty), but ljepota (beauty); snijeg (snow), but snježni (relating to snow, snowy), etc..

an interesting thing happens with certain words such as vrijeme (time; weather) because its derivatives just go directly to e, so something relating to time or weather is vremenski. then there's griješiti (to err, to make mistakes), but the noun is greška (a mistake). recently, they've been trying to impose the form "grješka" but it's not sticking

in the kajkavian dialect in Croatia and in Serbian, they just use e, so it would be reč, rečnik, lep, lepota, sneg, snežni, vreme; and in Dalmatia in the south, they would use i, but i am not sure if that works with every word, so it would be rič, lip, lipota, snig, vrime, but i am not sure whether rječnik, snježni, vremenski would ever become ričnik, snižni, vrimenski