Only noticed this today, while working on jquery translation hacks:
A shop set up with discounts by quantity for the default category (all products have the same discounts: 10+ -$1, 20+ -2$, 100+ -3$).
Ordering 1 unit of product A, 11 units of product B and 21 units of product C, all FoxyCart pages display one discount amount for all products, i.e. "discount -2" for all products, even though product A has no discount, product B has -1 discount, and only product C has -2 discount.
The subtotals and totals are calculated correctly, luckily.
Is this a bug in FoxyCart, or did I do something unwise with jQuery along the way?