The correct pluralization for "stone", the unit of weight, is "stone". A person weighs 10 stone, not 10 stones. Stones is only used to refer to a group of rocks.

I thought only the exceptional case mentioned was wrong, but if it is true what you say, then the exceptional case is right and all regular cases are wrong.
Maybe I should raise the priority :)

I'd say first of all that I'm not English native... but I'd like to ask if this property is shared with all the units. 10 meter or 10 meters? 10 second or 10 seconds?

No, it's a quirk of a few specific ones.

English is a foreign language for me as well. I just noticed the inconsistency and may have drawn the wrong conclusion about which side was correct and which side was wrong.
Googling several units seems to show some controversy on the issue.

On http://en.wikipedia.org/wiki/Stone_(unit) you can read:
However the stone remains widely used in Britain and Ireland for human body weight: in those countries people may commonly be said to weigh, e.g., "11 stone 4" (11 stones and 4 pounds), rather than "72 kilograms" as in many other countries, or "158 pounds" (the conventional way of expressing the same weight in the United States and Canada). The correct plural form of stone in this context is stone (as in, "11 stone" or "12 stone 6 pounds"); in other contexts, the correct plural is stones (as in, "Please enter your weight in stones and pounds").

On http://en.wiktionary.org/wiki/pound you can read:
"pound (plural pounds) (sometimes pound after numerals)"

Anyway whatever position you choose, you should not write "1642 stone" when your inventory weighs 1 stone and "1642 stones" when your inventory weighs 10 stone(s). This is how the game behaves.

The game pluralizes stones because the stones that the game uses are completely unrelated to the actual unit (this is explained somewhere in the manual or FAQ). Regardless of the previous, though, it's still a bug, as Leen notes.

I think this was already fixed.