caveat

caveat    [Kav-ee-aht]

noun. a warning or caution

"Google unveiled Google Photos with free unlimited cloud storage.  There are a couple of caveats:  Photos can be no bigger than 16 megapixels and high-definition movies cannot top out at 1080p.  But this should not be a major concern for non-professional photographers."
"Scientific American's online survey of pet owners uncovered some interesting results -- to explain why dog and cat people love their animals.  There is an important caveat: the survey was informal, not scientific. In fact, rigorous research on the subject remains scarce."
"As the host of the 2015 Women's World Cup and a bronze medal winner of the 2012 Olympics in London, Canada has the best collective spirit to win the cup.  There is a caveat to this optimism: on the field, the Canadians haven't held on to the momentum from the Olympics.  Heading into this World Cup, their results have been mixed."

http://www.vocabulary.com/dictionary/caveat