Lol, I do remember how an upgrade of a JSON serializer (JSON.NET in that case) broke an application. So, there was a .NET service publishing events to a queue with a bunch of consumers - a bunch of Golang service and (our) Scala service. One day our service starts crashing cause it can't parse the payload. We asked developers of the upstream service if they had change anything and they said "no, and by the way, other consumers are doing fine".

Turns out, they upgraded JSON.NET and it started to name fields in different case. Golang parser didn't really care about snake_case, camelCase or PascalCase but whatever parser we were using in Scala at the time actually did.