What are some times you've made a little mistake that could have turned into a BIG DEAL?

When I was a junior in college I had this massive final project worth 50% of my grade or something coming up and I somehow got the due date COMPLETELY messed up and thought I had a week longer than I actually had. I found out about this an hour or so before the project was due. Fortunately my professor understood my long and profuse emailed apology and pleading for an extension-- he must've sensed the panic in my words-- but oh man that was bad.