But if we're talking about a database insert and there's a collision (and the uuid column has an unique constraint - as it should, since it's meant to be a prim

But if we're talking about a database insert and there's a collision (and the uuid column has an unique constraint - as it should, since it's meant to be a primary key), then the insert will not be successful. Your application might not care about treating this error, but the DB will report it.