Arabic keyboards lack many important special characters. Here is a list of the most important characters and signs which you can copy and paste.
The correct vowel endings damma, kasra, fatha in Arabic can be a headache. If you are careless, you could turn verse 9:3 of the Qur'an into blasphemy (kufr).
The و does not always mean "and". The waw of accompaniment (واو الحال). Such constructions are tricky as we need different words in English.
The plural of Mustafa in Arabic is difficult - because there is a weak letter involved. How do you form masculine human plurals with a possessive pronoun?
What's the function of وَ in the saying: كُلُّ عامٍ وَأَنْتُمْ بِخَيْرٍ? It is a dubious one. Most grammarians say that you don't need the وَ and that it would be of better Arabic style to skip it! Let's see why.