Jargon: When writing your CV you may believe that using industry-specific jargon is a good idea and that the person reading your CV will understand it but using acronyms without explaining them can be confusing to the reader and may deter someone who is not familiar with them.
Qualifications: A common mistake in a CV is to detail all your academic history - even from 10-20 years ago whilst also listing all your grades! This is not necessary. On a graduate CV or someone with limited employment history it may be useful, but for others, just briefly cover your qualifications - detailing your most recent achievement and list older ones. If you have vast employment experience spanning many years, the chances are a recruiter will not even read your qualifications.
Hobbies: Your CV is a personal sales tool. Any available space on a CV is limited and you want to use it to sell your skills and experience effectively. Everyone likes reading, listening to music, keeping fit and socialising with friends, so unless you have done something special - and relevant to the job you are applying for - leave it out!
Too many pages: All too often, employers will receive CVs that are too long! A good CV should be no more than around 2-3 pages (for graduate or more executive positions this may vary). The more there is to read, the less likely it is that all your skills and qualities will be noticed. Try to decipher between what information is relevant to the job you're applying for and what is not.