It's definitely a bug. It's often distributed systems timing where things are "eventually consistent", just not at the moment you indexed. It can also be manual sql clients or other applications connected but not coordinating with the app. Not that I recommend that microservice madness style of development, but I've had to support databases in those scenarios. Pitfalls abound if you want to enforce uniqueness only at the application layer.

In the worst case bugs, systems can hum along for years with silent consistency problems. Database columns that are assumed to be unique but aren't - the truth only shakes out when you CREATE INDEX. Then once you fix that, you've got to find why the app was doing it in the first place! Generally its better for the app to crash than to silently corrupt the database as it had been doing all along.