We recently ran our first civiEvent with online bookings (and quite complex pricing). I installed civiDiscount and configured two coupon codes for groups who should be free - the event organisers and the National Council of the parent organisation. I set the discount level to 100%, and while the coupons were registered as having been used, it did not change the price on the registration form at all. When I set one of the discounts to 99%, it worked as it should. Fortunately, all our payments were offline, but it meant we couldn't report reliable income figures.
From my reading of the documentation, a 100% discount should be possible, so is this in fact a bug?