mikefarmer/spree — comment on commit ef31aa2546427e9746e31f73a6ea5bf4c26fd6e3
File: promo/app/models/coupon_code.rb line 9
By mikefarmer:

They didn't have any ability to handle coupons that could equal or exceed the Order total so yes, that was a bug. They also would have refused a single non-combinable coupon as well :) (see line 44 in promotion.rb). This method is meant for things like shipping rates and since CouponCode is also an adjustment originator, it just doesn't apply.