Situation:

No taxes used

Product Price: 5.60 discounted to 3.348

It show up at 3.35 with 2.26 discount (first error. It should be 3.35 with 2.25 discount)

If you buy 8 of this product, the cart show a total of 26.80

If you check out, at step 2, order total show 26.78

It seems that this comes from the change in includes/classes/order.php occurred from 1.5.0 to 1.5.1 (around line 573)

Not investigated the rounding error in the price showing in product page (I gues it will be in listing too).