OK, I went back and checked and here is the verbatim fine print on the most recent 20% off super coupon (comments in parentheses are mine):
Use this coupon to save 20% on any one single item puchased when you shop at a Harbor Freight Tools store. Cannot be used with any other discount or coupon (I would imagine "any other discount" includes a sale item??). Coupon not valid on any of the following: gift cards, Inside Track Club Membership (However, I think we should all chip in and get one for Dargo!), extended service plans, Compressors, Generators, Tool Cabinets, Welders, Floor Jacks, Campbell Hausfeld products, open box items, Parking Lot Sale items, Blowout Sale items, Day After Thanksgiving sale items (including cranberry sauce), Tent Sale items, 800 number orders or online orders. Coupon not valid on prior purchases after 30 days from original receipt (actually I never thought of using one retroactively). Coupon cannot be bought, sold or transferred. Original coupon must be presented in-store in order to recieve the offer (so they don't take xerox or pdf printouts anymore??). Valid through 5/20/11. Limit one coupon per customer and one coupon per day (has the one coupon per day limit ever been enforced??).