I'm only guilty of doing this once in 4+ years of tandeming. We were doing a day of riding over Swiss Alpine passes with some friends. After the photos at the top of the final pass (St Gotthard), everyone was ready to go, and so was I - until I was informed by my stoker that I wasn't on my single bike, and maybe I should wait for her to get on instead of leaving her at the top of the mountain. WHOOPS! Fortunately, I had only managed about half a pedal revolution before she informed me of my mistake, so I wasn't in TOO much trouble.