Q. This is an unintended break in a network diagram of Project schedule?
A. A basic scheduling rule is that every task should have at least one predecessor and at least one successor. The obvious exception is the project start milestone, which has no predecessor, and the project complete milestone, which has no successor. When a task lacks a predecessor and/or successor, in other words, a task missing required logical relationships, has a 'hanger,' which is an unintended break in the project network diagram. The problem is that the forward and backward pass calculations will be incomplete and possibly wrong because each hanger results in a roadblock for critical path method calculations.
[Source - Internet]