Early in my first career, once I had aircraft pretty much figured out, I accepted a mission to collect soil samples in the Highlands near the north pole. I build a small one-kerbal Mk1 plane and flew there, piloted by Ace Pilot Stelicca Kerman. After a few circles, I found a flat spot to land within walking distance of the Alpha site, so I landed, got out and walked to the target zone. I took the sample and started walking back. Then I had this conversation with myself:
"Hey, is the plane moving?"
"It looks like it's moving slowly backwards."
"It's moving faster! You set the parking brake didn't you?"
"..."
"RUN! We can still catch it!"
Stelicca ran as fast as he little legs could carry her. She almost reached the ladder just before the plane pitched over the edge of a cliff and smashed to bits 70m below.
I recovered Stelicca and what was left of the plane, but I still had two soil samples to get. I built another plane and flew again to the Beta site. Had to land on a hillside for this one but if I pointed the nose uphill and set the parking brake, the plane stayed put. Stelicca got out to collect the next sample. as soon as she's off the ladder, the plane tips up until it's resting on its tail. it doesn't roll downhill, but the ladder was dangling a few meters off the ground, out of Stelicca's reach. I had to recover them separately again and fly back again. Three planes for three soil samples. Not a major loss, but definitely a facepalm moment.