This one has bugged me since I was a kid. Look on any Band Aid package and you'll see this pointless pledge. I mean, we all know they can't guarantee an open bandage retains its sterility. So why the guarantee? It's the Schrödinger's Cat of product safety: you can't know if your Band Aid is clean without collapsing the waveform.
I've got a few more to propose.
Mac OS X: Guaranteed not to crash unless computer is running.
Windows: Guaranteed not to crash unless you're alone or with someone.
Claritin: Guaranteed effective in the absence of allergens.
Taco Bell: Guaranteed E. Coli free unless opened or eaten.