On of my worst MTB moments came out great, but was all about sheer terror when it happened:
Descending a fairly steep fire road in the mid-90's on a fully-rigid '89 Specialized Hard Rock. Fallen tree across the trail right at the bottom of the hill, but plenty of warning. Funny popping noises from the front end during the descent. I slowed from an impressive (rigid bike) speed, popped the front end up to clear a root as I steered around the tree.
The front end came up light...
reallylight...then I noticed my front tire rolling smoothly over the ground ahead of me as my forks rose and rose
Gravity helpfully reversed the forks' upward progress just as I thought to pull back hard and loop out like when you over-power a wheelie. Damn gravity. Forks hit and over the bars I went. Came out of the toe clips by reflex. Childhood Tae Kwan Do training and some PE practice saved me from splatting on hard clay trail with mixed rocks and roots. I took the impact on my forearms and shoulder with my head tucked and rolled out, slid a bit and missed being injured when the bike hit me.
As I'd been in the lead, my companions had quite a view of the fiasco, and were amazed that I came through uninjured, save my pride and a few scratches. I was flatly stunned, but so embarrassed it took hours for me to realize how fortunate I'd been.
It turns out the bearings in my front hub had frozen. Popping noises. It's just possible that I installed the safety-catches improperly, such that the torque of the frozen bearings spinning the axle disengaged them. By the bottom of the hill, the only thing holding the front tire to the forks was gravity and force of habit...the walk out sucked, but it wasn't too far. I shudder to think of how it might have ended had I lost the wheel just a bit earlier, during my downhill bomb with 2 riders right behind me
Watching the tire roll over the ground as my forks came up is definitely a moment I'll never forget and really, really hope to never repeat.