A man playing a horror game on the HTC Vive was so immersed that he ran and smashed his face into a wall, requiring stitches. Here's the video.
If you haven't used a VR headset (a real one -- Cardboard doesn't count), it's hard to understand how real it can feel, as evidenced by many popular VR reaction videos on YouTube. Here's a video that shows just how intense the experience can be.
A man was playing Brookhaven Experiment, a horror shooter (available on Steam for the HTC Vive and Oculus Rift). At one point, he ran out of ammo just as a mutated creature lunged at him. Gripped with terror, he instinctively turned around and ran, smashing his face into a wall, and punching holes in the drywall with his face and his knee.
Here's the video:
The man required stitches but was otherwise fine. The HTC Vive headset survived as well.
The video was posted by u/Stradequit.