This bug is fairly old but still exists in the 4.2RC versions, so I'm posting it here since it is quite severe:
Random IRQ_NOT_LESS_OR_EQUAL, BAD_POOL_HEADER & MEMORY_MANAGEMENT BSOD's occur on Win hosts when shutting down a VM or saving its state, while at least one other machine is running.
Tested on both XPsp3(32bit) and 7sp1(32bit) on two different computers with all 4.1 and 4.2 versions.
To reproduce: Start 2 VMS running only are bare live linux. Let one running, while saving the state / resume the other. Repeat. It might take upto 10 tries until host BSOD occurs.
Bug Report with dumps and logs: https://www.virtualbox.org/ticket/10867
Relating? Bug Report: https://www.virtualbox.org/ticket/10276