Long standing bug here which I thought I’d check if it’s been fixed but it hasn’t.
Load up the instrument version of Loopmash, get something playing, go to the Performance Controls, select a control with the left mouse button and with the left button still held down, press the right mouse button. Release both buttons.
Net result is the control gets locked and it’s impossible to unlock it short of reloading the instrument.
I hope that’s not “as designed” and me not reading the manual correctly!