Engineers will do what engineers will always want to do, they want to see how things break, and sometimes they manage to fix it.
Most engineers with more than a few years of experience know better than to directly try a stunt like that in production, but they do often get there through painful personal experience. I just wish I hadn't been one of the many that got to suffer from that engineer choosing to learn at that particular moment!
If this was 10 years ago, this was also the era when any real customer could write any ARN (resource identifier) they wanted including commas due to no expected standardization across services. This could have been trivially triggered by a customer.
Just a matter of time if no one is escaping commas in billing files.