Two separate systems each need exclusive use of two different resources. Each of them picks up one of the resources at roughly the same time, and then attempts to get access to the other resource. Bad development practices mean that each system won't give up their own resource if they can't get all of what they need, so the two systems remain at a standstill. The effect is that the resources that these systems are holding remain locked up in perpetuity, and the threads that are trying to request said resources remain effectively locked.