This was an unexpected failure: Karen and I had just finished our ride on Saturday and as she was dismounting the bike the stoker's seat fell off! It turns out that the head of the seatpost is just pressed onto the post and it had worked itself off. It's a good thing we were at the end of the ride instead of being stranded in the middle. What scares me most is what could have happened if it occurred while we were riding.
This was a Tamer PivotPlus. We got it with our Rodriguez tandem and it has about 2,500 miles on it. I took the post back to R+E and since they didn't have another Tamer in 25.4mm, Scott gave me a Crane Creek ThudBuster to try. I hope that Karen likes the ThudBuster because it looks more trouble-free to me: The base of the head is a machined part of the post - it is not pressed on - and there are fewer moving parts. I have heard of cases where the elastomer insert ruptured, but I hope that sort of failure would not be catastrophic. The Tamer design is always under spring tension and depends on a friction fit to keep the head on.