[cyberax]

Region (and even availability zones) in AWS are independent. The regions all have overlapping IPv4 addresses, so direct cross-region connectivity is impossible.

So it's actually really hard to accidentally make cross-region calls, if you're working inside the AWS infrastructure. The call has to happen over the public Internet, and you need a special approval for that.

Deployments also happen gradually, typically only a few regions at a time. There's an internal tool that allows things to be gradually rolled out and automatically rolled back if monitoring detects that something is off.