One can be in violation of trademark infringement for a number of reasons. The most common incidence is when an individual uses a brand name in commerce identical or similarly confusing to a trademark owned by another party. The United States Patent and Trademark Office would send an Office Action letter to a trademark applicant if the new application posed a likelihood of confusion to the consumer.
However, many times, trademark infringement occurs innocently, where the infringer was simply not aware that they were in violation of another party’s trademark. The infringer must comply with a Cease & Desist notice or face legal proceedings.
Over the weekend, two men from Mississippi were in violation of trademark law when they attempted to sell wood as Toshiba brand laptops. More on the story here: http://blog.trademarkia.com/2010/08/02/men-arrested-for-trademark-infringement-for-selling-wood-as-toshiba-laptops/