When is no visible circular dependency created by the user, there is no predecessor set up, only a successor (or the opposite), and trying to add a dependency that's not set, an error is thrown:
PRJ-07600: Circular Reference Error
Release : Any
Note: At this point there isn't an quick way to pinpoint which exact dependency is circular - the Project Manager will have to check the chain up and down one by one to see the circle especially if the projects is very large with complicated dependencies
If no dependencies on the task at all (predecessors or successors), but still having the issue, check out Task Dependencies throwing a PRJ-07600: Circular Reference Error (without any dependencies)