It is possible to create predecessor relationships that are not possible. Consider:
Task C is supposed to finish at the same time as Task A, but it can't because of other constraints.
Currently, SmartSheet silently ignores a condition like this. This happens to me when trying to build a project around external constraints - I can create immovable milestones and schedule tasks that are supposed to finish before a milestone, but since SmartSheet ignores errors, I have to be vigilant to make sure that the predecessor relationships are actually respected.
A predecessor relationship that can't be met should be flagged somehow - make it turn red in the predecessor field, or turn the whole field red, or make the predecessor arrow red, or pop up an error dialog. Anything but just dropping it on the floor!