Qb3 should give an error if

There are more than one Q on the board and they can move to b3.

That will be an interesting check!

I assumed dragonchild meant what s/he said :).

I said what I meant and meant what I said. I even gave an example.

  +---+---+---+---+---+---+---+---+
8 |   |   |   |   |   |   |   |   | 
  +---+---+---+---+---+---+---+---+
7 |   |   |   |   |   |   |   |   |
  +---+---+---+---+---+---+---+---+
6 |   |   |   |   |   |   |   |   |
  +---+---+---+---+---+---+---+---+
5 |   |   |   |   |   |   |   |   |
  +---+---+---+---+---+---+---+---+
4 |   |   |   |   |   |   |   |   |
  +---+---+---+---+---+---+---+---+
3 | Q | x |   |   |   |   |   |   |
  +---+---+---+---+---+---+---+---+
2 | Q |   | Q |   |   |   |   |   |
  +---+---+---+---+---+---+---+---+
1 |   |   |   |   |   |   |   |   |
  +---+---+---+---+---+---+---+---+
    a   b   c   d   e   f   g   h
[download]

How would you indicate that the queen at a2 moves to b3? Q2b3 doesn't work, and Qab3 doesn't work. You have to say Qa2b3. And, before you say this is far-fetched, the notation isn't for normal games. It has to describe every single possible game.

Update: It seems that your link to wikipedia even describes this situation, and I quote:

It may be necessary to identify a departing piece with both its file and its rank in unusual configurations (e.g. the player has 3 queens or 3 knights on the board).

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Then there are Damian modules.... *sigh* ... that's not about being less-lazy -- that's about being on some really good drugs -- you know, there is no spoon. - flyingmoose

I shouldn't have to say this, but any code, unless otherwise stated, is untested

You said

Remember, Qa2b3 means that there are three queens, potentially at a2, a4, and b4

Now in your reply you are showing a queen at a2, a3, and c2, and in this case there's no argument that Qa2b3 is unambiguous and necessary.

Please understand, I wasn't disputing the occasional need to use both rank and file, just querying how you could deduce the existence of three queens from Qa2b3

So forgive me if I misunderstood you, but perhaps you can see how my confusion arose.