Is it allowed to ghost someone without a warning for not being able to reply to ATC just once
It depends on the situation.
OMAA right now , kinda busy
If you were ghosted, you should contact the atc on a PM and he would explain his side
What if it was a mistake
If it was a mistake the ghost can be reversed
A guide to appealing ghosts is a good topic to read if you are appealing/before you appeal
It really does depend on the situation. If it can be prevented, a warning would most likely be issued. For example, not following an instruction, duplicate messages, or accidentally lining up for the wrong runway on final.
If an aircraft, gets impatient and taxies through the 20 aircraft patiently waiting to take off during an FNF then tokyo drifts onto the runway at 50 knots in front of someone on final, or takes off from the gate - you wouldn’t expect a warning would you? No one wants to see that on expert server.
Depending on situation, yes. For further information, contact the controller.