Realising - a split second before it happened - that the helicopter I was student (not flying) in was about to undertake topiary on an oak tree.
And yes, that was expensive.
Sitting in my machine at the hover awaiting take-off clearance, watching an EC120 student / instructer combo fluff then save an auto from about 50m distance only for that other machine then to "dig-in" and nose-over. I couldn't figure out where to hide from the shrapnell that would result from the blade impact. It didn't happen - blades came within a foot of the ground, though.