Water droplet hitting the top of a spout from a previous water droplet (sounds simple when you say it like that!)
Maybe tried to get a little too close and sacrificed DOF too much.
Strobist Info :
Vivitar 285 set to minimum power (with flash and vari-power modules set to 1/16th) 2sec exposure with flash fired at water and at a rainbow coloured sheet behind drops. Low angle