It does happen, but it's pretty rare in flight.
Far more common (and something that leads to often serious injuries) is a slide deploying on the ground because the doors were not set to manual properly after landing. It shouldn't happen - there are very clear mechanical indicators showing if the door is in manual mode (you need to physically pull the handle to deploy the slide) or automatic mode (slide automatically deploys the instant the door is more than 10 degrees open) and crew are supposed to cross check each other, but there are a few cases per year. A dispatcher was killed at Heathrow a few years back when a slide deployed in his face, knocking him off the accessway. The fall broke his neck.