This will come as a surprise to many of her adoring fans, but there are a lot of people out there who don't particularly care for Katherine Heigl. The actress has never been in legal trouble throughout her life, and nor has she done anything in her personal life which has attracted negative tabloid attention. The dislike of her appears to stem from a reputation she has for being a 'diva' on set - she walked away from 'Grey's Anatomy' while under contract, forcing the show to be re-written. She also gave scathingly negative interviews about her movie 'Knocked Up,' after being paid handsomely for appearing in it.
Based on the above, it might be true that Heigl is hard to work with. She does make up for it in other areas of her life, though. She's attached to several charities; especially those who support animals, such as the Best Friends Animal Society. She's also been honored by the American Cancer Society for her publicity work on their behalf.