Garageband is not playing the correct instrument that is selected for a loop or when using musical keyboard.
For some reason, whenever I had selected one of the guitar MIDI instruments in Garageband, I get a sound like a xylophone, but when I click on the loop, it plays normally. After I drag the loop into the editor window, it plays the incorrect instrument, even when guitar is selected as the instrument.
This is a bizarre bug I can't seem to get around.
Mac mini (Mid 2011), Mac OS X (10.7.2), 4GB DDR3