BBI had the same problem...A customer had emailed complaining of the shopping cart defaulting to "cart empty" after each input. The problem has been corrected within the past few minutes.
Yes precisely, just before that, the discount codes did not discount the price but instead added to the price. Glad it's been fixed.
What happened was that one of the db slave servers fell behind but did not fall out out of rotation (which should have happened automatically), so buyers who were hitting that slave were unable to see the carts they had just created on the master, that's why they started getting cart verification failures.
In your case, when you applied the code, you'd removed an item from cart as well but item did not get removed (due to the lag) and cart included the unremoved item and its price.