Bill and I were on his tandem, on our way back to Parker from the Park Meadows Mall area. We were riding on the E-470 Bike Trail, and Bill was on the front of the tandem, as usual.
At one point on that trail, you have to take a sidewalk for a short distance, alongside a road which passes through an underpass to get to the south side of the trail. As you go under the highway, the slope beneath bridge is covered with a concrete slab.
As we rode along, at about 15 mph, Bill suddenly veered to the right and up the concrete slope. Then he turned back to the left, and we followed a parabola back to the sidewalk.
This happened so fast that I didn't even have time to brace myself. As we came off of the concrete slope, onto the flat sidewalk, there was bit of G-load applied to the saddle I was sitting on. The seatpost, which was extended about 250mm out of the frame, could not support the extra load, and it bent at the point where it entered the frame.
Bill ended up with a seatpost with about a 40-degree set-back in it, because he didn't warn me about the maneuver. It was a learning experience, I suppose, because he always let me know what was coming, after that.