Putting the wrong parent on the form
Why it applies to you: Hannah's parents are divorced, and the federal form and a CSS Profile college can define "the parent" differently. Naming the wrong one can quietly attach the wrong household to the application.
What to do: Confirm the custodial-parent rule for each form before you start, and use the same parent consistently. When a college asks for the noncustodial parent too, plan for that as a separate step.