This shows a simulation of 15 pendulums with periods from 41 to 55 oscillations per minute, started in sync. As a group, they change from moments of beautiful harmony to what appears to be chaos. Each pendulum is, of course, unwavering in its oscillation period until they "find" one another again.
See the YouTube video of the real pendulums here.
Wolfram Demonstrations Project
Published: January 21 2014