Stop signs or lights that end in an uphill.
Seriously, after ten years, I still cannot maneuver this. Even a teeny, tiny uphill, I cannot just push off and go. You can give me an 18% grade to climb instead and I would be happier.
I know what to do. I just can't do it. There have been quite a few embarrassing moments because of this, which entail scooting across intersections with the unclipped foot, getting caught on the saddle with my shorts, and the best one, actually getting going, but my foot that is unclipped gets in a weird position, the pedal is in the up position and I almost crash while I am riding with one leg.
All of my rides are planned around these types of intersections.