We are still an inexperienced team (~300-400 miles under our belts in about 2 months) so it happens to us probably more than it could/should. My wife and I did the same 100k ride last Sunday Rick mentioned and we have been having trouble with our FD not shifting reliably into the small ring if there is _any_ tension on the driveline at all (which really sucks and is something I have been working on fixing--the old reverse pull Sun Tour Comp V has a very loose cage so my next step is to swap it). As long as I go to the small ring way early we're fine, but if I miss the opportunity we're pushing hard or stopping (and I really hear it from the back because "stoquette" does not like to stop on hills). There was one short, but super steep, climb right before the lunch stop that I totally underestimated. By the time I started seeing a bunch of single riders walking up the hill it was too late to catch the small ring. I knew we were not going to make it in the middle ring (a 42) so we stopped and shifted to the small ring holding the rear wheel up. I had my doubts about restarting. We tried twice and aborted both times. I said we need to walk this one. My wife said before the ride she didn't want to walk up any hills when I told her we might have to. I saw a level dirt driveway and realized we could start there so we did and we made it the rest of the way up, passing quite a few walked singles at a half a MPH or so faster than walking. We actually did have to bail out and walk one more time for a few yards at the top of Lewis Rd., another climb I totally underestimated when looking at the route. After those two climbs we knew the rest of the route from doing training rides out that way so things got much better.
A big part of the problem for us is that my stoker insists on not starting "the proper method" and we do okay the vast majority of time with both feet down together (IP).
@Rick, sorry we didn't connect last Sunday. I wasn't able to PM due to low post count and figured I would just look for a daVinci (I think I saw a black daVinci at the Aptos High "dinner stop"...yours?). My wife's first ever organized ride and my first in almost 40 years. We had a great time and my amazing stoker is already asking when we'll do it again. I added a photo of us that a friend from our spin class took close to the top of Hazel Dell in the happy tandem couple photo thread below. Hope you two enjoyed the ride too!