One of the most serious mistakes that students make on the TOEFL Writing exam is not paraphrasing properly. It may seem ridiculous (why say it in your own words if it's already been said?), but it's important to remember that students in American and Canadian universities do a LOT more writing than in Spanish universities. The ability to write clear, accurate prose is fundamental, and part of that ability is to be able to say someone else's words in a different way - which allows you to compare and contrast ideas, thoughts and possibilities.
Problem is, a lot of students now think that it's acceptable to avoid this and to copy information directly from the Internet. This story from The New York Times explains how widespread (=common) the problem is, and why it's such a serious problem for universities.
(Of course, TOEFL students who are well-prepared won't have this problem!)