Attempt stacking 'CAN-BUS' and 'USB Host' Shields on Mega 2560
I think that there is an hardware bug on the USB Master shield:
IC3, the 3-state buffer which converts MISO to 5V, is always active...
The enable pin of this 74LVC1G125 (pin 1) should be connected to the pin ss_3V3.
Then you can stack this shield with another one.