"My mom is up all hours of the night pacing, like she's haunted - a ghost with an unnamed grudge." - Nate Expectations by Tim Federle. Hearing this quote while listening to the audiobook "Nate Expectations", specifically the line "a ghost with an unnamed grudge," made me want to name 10 possible grudges that might cause a ghost to haunt someone.

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1. S/he was murdered, and their murderer still roams free.
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The most obvious reason for a haunting.