I bailed out into a rosemary shrub the other day when a father teaching his very young son how to ride a trike got in my way despite my approaching them at the very minimum speed required to remain upright. (On a street w/a cul-de-sac and a gated ramp into a park with paths leading to major streets.) I think the kid got distracted by the shiny bike and just drifted over and stopped right in front of me. You do whatcha gotta do. Of course, this wasn't a panic stop so much as a bemused slapstick stop.

In terms of real panic stops, this Sheldon Brown page might be helpful reading.