Usually there's a tacit assumption of how the system works, how the users are using the system, or something else about the system or the environment that cause

Usually there's a tacit assumption of how the system works, how the users are using the system, or something else about the system or the environment that causes that - it's not that the answer wasn't known, it's that it was assumed to be something it wasn't and nobody realized that was an assumption and not a fact.