*** PROGRAM LOGIC ISSUE ***
If you enable [Show quantity selector as input textbox] in Admin -> Appearance there are two different behaviors in Product page and Cart page when updating the quantity.
1) Product page - let's take as an example XC Gold+ out of the box. In MicroFly Tiny R/C Hovering UFO product page input a number of 1001 pieces for quantity and click [Add to Cart] button. A window message comes informing you "Sorry, the maximum order quantity for this product is 1000." (by the way I have seeing different types of windows inside XC: JavaScript, jQuery, ... - once and for all make only one type jQuery fading windows). Ok I order 900 pieces which is fine.
2) Go in Cart page and change the quantity from 900 to 1001. Click Update. You will see inside the input box value 1000, but no window is coming to inform you about why only X products you can order. If you are a moron you could not understand what is going on and continue changing the quantity
Both Product and Cart page should deal in the same way with quantities especially when a customer asks more products than the store could offer!!.
Professional carts will deal in such of situations like inside the attachment. If you want 500 and I can offer only 200, you can backorder those 300 (I don't know if BCSE Backorder/Preorder is acting like this - since no demo only pictures like what offer many 3rd party developers).
But for the moment we have to dream getting such of feature. Ok but please use two jQuery windows to inform the customer about quantities, which is not a big deal.