Why is this an issue?

When two locks are held simultaneously, a wait call only releases one of them. The other will be held until some other thread requests a lock on the awaited object. If no unrelated code tries to lock on that object, then all other threads will be locked out, resulting in a deadlock.

Noncompliant code example

synchronized (this.mon1) {  // threadB can't enter this block to request this.mon2 lock & release threadA
	synchronized (this.mon2) {
		this.mon2.wait();  // Noncompliant; threadA is stuck here holding lock on this.mon1
	}
}