Imagine showing up for a job interview a little tipsy, wearing mismatched clothes, and then talking for the entire time about things unrelated to the position you're applying for. Most people would never do that -- yet applicants often make the equivalent of those mistakes on their cover letters.
Your cover letter is essentially a pre-interview. It's a tool you can use to show a potential employer something of who you are in ways that don't fit neatly into a resume. If you waste that opportunity, you may never get an interview, let alone have a real chance at landing the job.
1. You spend the whole time being silly
It's fine to show some personality, or even to have a little fun. Going full absurdist may make the person reading your cover letter laugh, but a laugh isn't the same as someone wanting to interview you.
Don't go too far off the rails. Focus on why you're a good fit for the job, and if your fit is indirect, make your case but be careful to connect the dots. In most cases, if you don't write a traditional cover letter and opt for a silly story or some other form of comedy, your resume is going in the trash (and you'll never know if you got a laugh or a groan).
2. Don't talk about another job
Your cover letter should explain why you want the job being advertised. It should not expound about how your real goal is to be a mountain climber, a poet, or a sheepherder. The people doing the hiring want to think that you're passionate about the position being offered.
It's OK to confess to a hobby, especially if it relates to the work. It's not a great idea to express long-term goals that have nothing to do with the position being offered.
3. You go full-on generic
If your cover letter talks about how you the skills to succeed in any job, the hiring person won't be impressed. The same is true about talking about "drive", "passion", or how hard you work.
Those are all great things, but you need to address the specific job being hired for. Make it clear that you wrote a cover letter for this position. Cite questions raised by the job ad, and explain why you specifically fit.
It's great that you're a hard worker, but relate that in a specific anecdote that ties to what's being asked for in this job ad. Remember that every person sees themselves as qualified, and it's your job to make yourself stand out with why you'll do well in the role being hired for.
Take the time to shine
It's OK to have a rough cover letter that you adapt for every job you apply for, but make sure you adapt it. Many job ads ask questions that aren't answered on your resume. Find a way to address those things in your cover letter.
In many ways, a cover letter is a test as to whether you can follow directions. The company has asked for specific things, and if you ignore them they may well ignore your application.