Moving from the big picture to the small picture, I gave the current auto-fill behavior of the browser a little more thought, and decided I consider aspects of the current implementation to be a bug.

I consider features that can easily cause you to lose data to be bad and wrong, and the current auto-fill behavior falls right into this definition. The current auto-fill behavior will put data into any field it recognizes. In doing this, it will replace existing data if there is any – often without the user even noticing. This is what caused the problem with Tasks Pro™, the browser overwrote the existing username of a user with the auto-fill information it had.

I checked and there is already an open bug for this issue, please take a moment to vote for it.