1. If the content of your work is available anywhere else in any medium. For example, a how-to about how to install Virtual Box under Ubuntu. There is a lot of material available on the subject. Compiling and re-writing the already available information will be considered plagiarism.
2. If your work includes quotes that can be found elsewhere attributed to another writer without proper citation.
3. It is originally written by you but has already been published somewhere else.
4. Consists of too much third party information – from company press release or wiki or documentations.