I stumbled on a bug that i need some folks to verify, but it's pretty specific to the hardware i'm using. This is for Logic Pro 9.1.8
I'm using an Apogee Symphony I/O, and also an Access Virus (two USB interfaces, basically).
A) Load up a song in logic, and make sure it's playing.
B) While the song is playing, instantiate the Access Virus plugin.
C) Logic will prompt you, saying it found a new device "Virus TI", do you wish to use it?
at this point, it appears that everything connected to the USB hubs stops responding. the keyboards, etc.. Spinning wheel of death occurs. I can switch applications by clicking on them (I have two LCDs, and use the trackpad which is bluetooth). however, I cannot enter text, or click on any dialog boxes (for example, clicking "yes/no" in that "do you wish to use it" box in logic. I also cannot cmd-tab between applications. The song continues to play just fine.
I had to hold the power button down to force the computer to shut down. I'm gonna post this on the Virus TI forum as well, but because it was related to logic detecting a new hardware device for use with I/O, i figured it was relevant to all Logic users that use a USB converter of some type. Meaning, anyone who has more than one USB interface (say, a Motu Ultralite and an Apogee Mic). So, try playing a project through your default interface, then connect some other USB interface and see what logic does while the song is playing.