There is no normal scenario in which a controller is allowed to on guard an aircraft that is tuned in to another active frequency.
> [5.2.2](https://infiniteflight.com/guide/atc-manual/5.-airspace/5.2-on-guard-procedures#5.2.2) — Aircraft should be given a reasonable amount of time to tune into the frequency after being “on guarded”, in addition aircraft must not be ‘on guarded’ if on another active frequency.
Think about from the pilot’s perspective. Handoff to Tower then cleared to land. They’re on final, getting ready to land, then they get the on guard warning.
“N623KB you’re in an active airspace, please contact Philadelphia Approach on 126.6“
Panic time. As the pilot, should I stay with Tower? Should I switch to Approach on my own without asking for a frequency change? Should I request a frequency change from Tower? Am I going to get reported if I stay with Tower? The answer to that last question is no, you would not be reported.
If you’re the pilot in this scenario you’d ignore the on guard and continue to follow all instructions.
IFATC controllers on the expert server have the ability to message each other on Discord, while controlling. They’ll chat in a voice or text channel back and forth during the session to coordinate. All they have to do is ask the other controller.
You’d ignore because one, controllers can’t on guard aircraft on other active frequencies, and two, it could have been a mistake.
Two double taps on an aircraft with misplaced fat finger, a controller can accidentally on guard a pilot. This can happen, I have done it a few times.
The only unusual scenario where you could on guard is if the pilot switches from your frequency on their own. As the pilot, it’s crucial that you don’t switch frequencies on your own. Always ask for a frequency change when it’s appropriate.