It does happen. This guy recently died when he didn't make the route he had planned. This is not the video, but it shows the place where he had his accident.
If you watch the video, you can see the big cutout to the right. He tried to fly through that instead of going around to the left, but didn't make it. He had a masters degree and did research with wingsuits in school. So things can still go wrong even if you calculate it and take many precautions. But it is what anyone risks when they take part in an extreme sport they love.