Although the title might sounds subjective, let me prove firstly that it isn't so: If the title would be
Question. Which common grammar mistakes do Russian native speakers studying German as a foreign language make?
there is an objective (perhaps not complete, but statistically speaking right)
Answer. They tend to omit or forget the article, to mix determinate with indeterminated articles (and the beginners forget the verb sein as well and only juxtapose).
I guess the same applies for another Slavic languages, but since I don't speak other than Russian, I won't say anything about them. q.e.d
Back to the question. It refers to two aspects
Phonetics. Beyond the famous
Ich komme aus Espanieninstead of Spanien, which other phonetic mistakes have you detected from Spanish-speaking people. It could help people to lose their accent (btw. Ich komme nicht aus Spanien)
Grammar. Grammar structures particularly difficult to Hispanics.