Fencepost Mistake

Common Mistake
Let $F$ be a fence of length $n$ units, with fenceposts at either end and spaced $1$ unit apart.

How many fenceposts are there?

To answer $n$ is to make the fencepost mistake.

There are $n + 1$ fenceposts.

Also known as
This is usually seen as fencepost error, but prefers to reserve the word error for its context in unavoidable inaccuracies in measurement.