(This is on OS X 10.6.
Two or three times now, something has happened while using the Arduino development package that's just odd. It may be related to unplugging/plugging the adapter while the Arduino app is running, or maybe disconnecting while the Serial Monitor is running; I don't know, but sometimes after the disconnect/reconnect, when I try to download a sketch or open the Serial Monitor, I get the message "Serial port '/dev/tty.SLAB_USBtoUART' not found."
Quitting and relaunching the Arduino app doesn't help. Further, the USB adapter is no longer listed under Tools/Serial Port, and it is not on the list in /dev; unplugging/replugging the adapter won't bring it back.
Attempting to unload the driver fails because it "is in use", even though I have killed the Arduino app.
So far, the only way I've found to get things running again is to reboot the Mac; very annoying.
I have tried to come up with a reliable way to cause the problem, but haven't been successful so far.
Any suggestions about how to eliminate the problem, or reliably replicate it?