Crowd synchrony

in symb(i/o)tika's video collection you can find a short film about the unexpected sway from side to side that emerged under the synchronization of pedestrians step... it is a very clear example where local action produces and emergent behaviour that then feeds back and drives that local action.

more on Steven H. Strogatz et al. paper "Theoretical mechanics: Crowd synchrony on the Millennium Bridge". published in Nature 438, 43-44 (3 November 2005).