Never throw behind the runner
Throwing to a base where nobody is running gives away a free base — and the out you had.
The rule: Before you throw, ask: is anyone actually running to that base?
The trap
Runner on third does NOT mean throw home.
With the infield back and less than two outs, that runner is standing on the bag watching you. A throw home gets nobody out — and the batter, the one runner you COULD get, is safe at first.
The fix
Take the out that's really there.
Throw to first. Out. The runner on third is still on third — exactly where they were. You gave up nothing.
Same idea
Run already scored? Don't throw home.
Single to right with a runner on third. That run is in before you pick up the ball. Throwing home just lets the batter jog into second.
You make the call.
Runner on third, one out, infield back. Routine grounder to you at second. Where is the play?