I've had a few moments while out riding that have startled either me or someone or something else.
The high engagement hubs thread made me think of a time when I startled a guy riding some single track which is tight with next to no room for overtaking, the oaks single track for those who know it. I or my bike must ride quietly as this guy I rode up to didn't realise I was behind him until I freewheeled my Hope hub which then went Buzzzzzzz. The noise made him freak out and he wobbled and almost ran into a tree. I apologised for startling him, we laughed and rode on.
Another time I was riding with some mates down some fast fire trail, Bee Nest Hill, and when we arrived at the very bottom of the hill we came across a lady with an exposed bottom! She was startled while relieving herself, must have had to finish up quickly!, and ran form the bushes red faced and sorting herself out while a few of her companions were laughing and joking with us about the embarassing event. Fun times indeed.
Another time while riding sweet single track, again very tight with shrubs whipping you as you ride along, a shrub's branch got wedged between my right index finger, which was resting on the front brake lever ready to slam them on if 'SNAKE', and the lever itself which instantly made the brake almost lock up and flick me into the bushes, I survived unscathed but startled and never have a finger on the brake lever in tight bushy single track anymore.
What startling moments have you had out there?