Just before dropping the flyer from a local HVAC company into the trash, I spotted their…
IRONCLAD GUARANTEE: We are confident that if your system is over 10 years old, you may significantly reduce your heating and cooling energy consumption – possibly as much as 25%**
There is no way you can lose. If these premium air conditioners and matching furnaces were not among the best on the market, I could’t afford to make such a promise.
**Individual results may vary.
What promise? I lost track after the third or fourth conditional qualifier there. Well, with an ironclad guarantee like that, how could I lose? 
I shudder to think they might guarantee proper installation, too.
To be completely honest, they should have qualified the heading too:
IRONCLAD* GUARANTEE**
…..
*may be tinfoil in some states
**subject to our approval