Serial port already in use after upload [Leonardo/OSX]

I have an Arduino Leonardo I'm using with my Mac running OS X 10.10.2 with Arduino 1.0.6

It previously worked fine. I have the serial port set to /dev/tty.usbmodem2021

I press upload, the tx/rx lights flash, then L flashes once, then rx flashes once and I get the following error:

processing.app.SerialException: Serial port '/dev/tty.usbmodem2021' already in use. Try quitting any programs that may be using it.

I've tried rebooting, unplugging/replugging. Any other ideas?