When something happens on a project where the timeline becomes out of sync, you'll sometimes see the following issue:  

The Error will state that a dependency loop was detected while processing the following tasks:

In order to fix this issue, you need to get to the last task listed in the error (the child task in this case) and remove the predecessor. 

Once done, you'll refresh the page and recalculate the timeline and the other predecessors will have been removed and the timeline synced back up without an error.