Misspellings are the most common mechanical mistake.
People rely on spell-check too much. Spell-check might be able to save you from spelling mistakes, but it cannot tell the difference when it comes to meaning. For instance, if you write "manger" instead of "manager." spell-check won't flag it. Other mechanical problems include verb tense shift and capitalization. It seems like when in doubt, job seekers will capitalize something just "to be on the safe side" but that just creates an error.