It's not, but it is not recommended, and it can also do damage to your fuel gauge on some vehicles.

For AVGas, when I was heavily in skydiving, we refueled the planes with the propellers still turning right after the plane lands and parks to pick up more skydivers. Saves wear and tear on the plane.

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Buh-bye.

Am I the only one tired of hearing about the gas hysteria? It's totally overdone.