alt Hacker NewsBack in 2020 I had a similar situation. Ended up charging $500 due to an overnight TPU training run using egress bandwidth across zones.
Google support was surprisingly understanding, after I explained the issue. They asked some clarifying questions. Then they said that they can offer a one time refund for this case.
Since then I was paranoid not to accidentally do it again. I don't know whether GCP would refund a second time.
GCP charging for interzone traffic is an interesting financial choice. They own all the infra and in many cases this is literally moving from building to building.