You can find plenty of advice on what to include in a cover letter. It's important to include detailed information on why you are the best qualified person for the job. However, there's also information that you should not include in your cover letter when you are applying for jobs.
Nina Scott, C.P.R.W. and Career Coach of Impression Resumes says "Strategy is a mistake which can often be a blind spot for job seekers." Job seekers may think that providing certain information can help them get an interview. However, including the wrong information can hurt, rather than help, your application for employment.
Here's what you shouldn't include on a cover letter, from Nina Scott:
Stating they are looking for a challenging position which will allow them to advance.
While it is impressive to be enthusiastic and motivated, stating a wish to advance up front in the cover letter may cause some employers to shy away, fearing high salary demands.
Mentioning salary requirements in the cover letter, without it being requested.
Again, not good. What if the employer wanted to offer more? Or less? Always let them mention numbers first, and do salary research online though Glassdoor.com or other salary websites. Knowing ahead of time what the competitive salary range would be local to the job opportunity will help to ensure salary expectations match.
Stating they are looking for a more challenging position (than their current one).
Looking for greener pastures does not sit well, because it plants a seed of concern the candidate is a job hopper.
When applying for a temporary job post, some candidates think it's OK to let the employer know the job would be perfect for them, because they are looking anyways!
I always share this is not a good idea, and to think of it from the employer's point of view. Even with a temporary job, the employer still wants stability for that period, and will select the most qualified candidate who they feel will stay on the job throughout. So although the candidate may be searching for a "permanent job", it's best not openly stated on the cover letter or in an interview, unless asked directly.