I spotted an old Google Maps Help thread that reported that in the past, a competitor was able to add a coupon to this guys business listing. So if you visited his business, you would see a coupon for another company. Clearly this guy was upset and it seemed like Google may have never resolved his issue (it is unclear).
It seems like now some business owners are having trouble deleting live coupons from their Google Local Business Listing. For example, Chautauqua Lodge has a coupon that doesn't look all that professional:
I wonder if it is still possible to add coupons to competitors local business listings in Google Maps.
Forum discussion at Google Maps Help.