"Meat and animal products are one of the number one causes of food poisoning," proclaims the web site for the Australian Institute of Food Safety. This is not a statement that makes any sense. By definition, there is only a single "number one cause" of anything. It didn't get to the top of the list by sharing that position with others.
#1 picture from Shutterstock
A cursory online search suggests this error is distressingly common, and not just in random blog posts or advertising copy. It also pops up in many a professional news report. Yet more evidence that not enough people proofread their work these days.
The lesson? If you know something is indisputably the "number one cause" of something, feel free to describe it as such (ideally with supporting evidence). If you don't know — either because you're too lazy to find out or because the evidence is contradictory or unclear — then use a less definite phrase, such as "one of the major causes". Precision matters, and accuracy matters too.
Lifehacker's Mind Your Language column offers bossy advice on improving your writing.