Calendar Girl
A friend left her Gmail account logged in on my computer, so we logged into her Calender and started adding new appointments. She lives by this calender and checks it several times a day. Approx 50-60 appointments were added all up spanning from the next week to 5 years in the future.
Appointments included back waxing appointments, very early run reminders and anything else ridiculous we could come up with. It took her about 6 months to realize who it was. She was really worried about where all these bizarre calendar events were coming from. Story credit: Reddit / Pulledporkpancakes