When talking about English, there aren't a lot of things which would peeve me. Maybe it peeves most if whole the text has been written with a lot of informal abbreviations or spoken language because I don't speak English as my native language and can't simply always understand informal abbreviations or spoken language. Besides that, most of dictionaries that I tend to use don't either know those abbreviations or spoken language words. In the worst cases I can't understand the text not at all.
When talking about Finnish, there are a pretty lot of things which peeve me. Maybe the most vexatious thing is compound words which are written apart. There are a pretty lot of compound words in Finnish and it really pokes my eyes if I have to read long text with lot of mistakes with compound words.
Despite of a language the lack of punctuations, capital letters or too long sentences/pragraphs vex me.
"On hyviä vuosia, kauniita muistoja, mutta kuitenkaan, en saata unohtaa,
Että koskaan en ole yksin, varjo seuraa onneain.
Vaikka myrsky hetkeksi tyyntyykin, varjo seuraa onneain.
Ja pian taas uusin hönkäyksin, varjo seuraa onneain.
Hei tuu mun luo, pieneksi hetkeksi. Puhutaan, varjoni, valkoiseksi enkeliksi." Pelle Miljoona - Varjo seuraa onneain