A mistake is an unintended or accidental error.
Approaching the year 2000, an amazing number of people on the Internet said that the year 2000 was not a leap year, and when I would try to correct them, they would argue about it. That was an error.
This year (2016) is a leap year, and I know that. But on February 29th, I completely forgot about it and I wrote "March 1st" on a check. That was a mistake.