What are the most common reasons for mispronunciation?

A common reason for mispronunciation in any language is not knowing the rules of phonics of that language. Or applying your first language phonics rules to a language you're learning.

For instance, Spanish is a phonetic language, meaning that the sounds of the language are straightforward. "H" is always silent. "ll" sounds like a "y" in English (depending on where you are). 

So the silent "e" in English can be a stumbling block. A native Spanish speaker would want to pronounce the final "e" in "make," if they don't know the rule.