The two most common tajweed mistakes made by non-Arabs are the timings of the vowels, and medd letters, and in the articulation points of the letters.  The first type of mistake has been addressed in the first of what will be, insha’ Allah many tidbits; please see the tidbit lesson link.  The second type of mistake, that in the letters themselves, is a major problem that needs to be addressed by all non-Arabs, whether they are Westerners, Europeans, Africans, from the Indo Pakistani subcontinent, or from Eastern Asia.  At the outset, the letters that occur in Arabic that are not common in other languages would be what one would imagine to be a problem, but in fact, there are letters in Arabic that are similar to other languages, but do not share the same articulation points with their counterparts.