Sometimes when I get an enemy down to zero health, it doesn't die until I hit it again. This happens to both bandits and wild Sylesties. The health bar says 0%, but it will still be standing. This doesn't happen very often though. Just every once in a while.

0% is probably a rounded down number.

For example:

4/1000 health is 0.4% health, so technically it's 0% health. But there's still 4HP that you didn't take care of. xD;