There are creepypastas here that are based on historical fact.
They_Remind_Me_of_Hell by Bloody Spaghetti is based on a historical incident. I don't know if it was one of his family members that participated in it, but this actually happened as described.
As for my stories:
I Was Born on a Farm is something that is well documented to happen. People ignore it, but it is accurate.
Both Happy Valentine's Day and The Fuzzy Easter Angel could have happened. The former is based on watching someone who I knew from Church who was a World War II bomber pilot slide into Alzheimer's. The latter is based on several famous cases.
Details were changed in I Have To Pee to try to make a better story, but the basic story is completely true and based on personal experience.
I was a witness to Meine Ehre heißt Treue!. I dramatized it, but there's a lot more truth to that story than you'd believe if you weren't there. I wasn't the protagonist, but I have know the protagonist for many years.
The story in Mutti Dearest is somewhat dramatized but true. That's my mother and my sister.
(Only my first four stories are autobiographical or partly autobiographical.)
Beetlejuice could happen. It is 100% scientifically possible.