Big-Wave Surfing
Gliding around at the bottom of a 100-foot wall of water doesn't sound particularly safe. But, when you factor in the rocks and environmental hazards that lay beneath the ocean where big wave surfing is possible, then you've got to consider that there are more dangers than just the water crushing you. Roughly 10 people die from big wave surfing per year, and you should not attempt it.
High-Altitude Mountain Climbing
This is a whole different beast compared to traditional mountain climbing. When you attempt to summit peaks like Everest and K2, you need to bring supplemental oxygen to sustain yourself so you don't pass out from altitude sickness. Hundreds of individuals have died while trying to climb Everest alone. Make sure you know what you're getting into before you give one of these summits a shot.