If authors leave out diacritics, should I correct them or leave it as written? While preparing a bibliography, I have noticed that some authors' journal titles have not used diacritics correctly. For example, some authors have written "Miguel Angel Asturias" in their titles rather than Miguel Ángel Asturias. 

Should I be referencing these titles exactly as they have written it, or should I correct them?
In my opinion you shouldn't change the title.

The purpose of your reference is letting your readers find the referenced work, so changing the title (even if it is to correct a mistake) may make this harder or even create inconsistencies in scientific indexing services.