Not writing one at all
An obvious one, but many people submit resumes without writing a cover letter. Don’t do this, a resume without a cover letter seems to lack seriousness about the application.
Copying the job description
If the job description says “fast moving and dynamic work environment”, don’t write that you want to work in a “fast moving and dynamic work environment”. That’s lazy. You will want to make it clear that you’re interested in their description of what they’re offering, but don’t copy and paste.
Spell check, and grammar check. Have at least one person with a good grasp of your language look the document over.
You should reflect your personality in the cover letter. One thing you want to reflect is your positivity, speaking negatively about previous jobs is a bad way of doing that.
Don’t bore your readers.