...That's what a client said to me.
After they'd nagged me to return a contract that they'd mailed to me without instructions. I'd filed the thing. Which led me to my new Unified Theory of Stupidity (trademarked, registered, weapon-strike, and blackballed wherever beer is sold).
Here it is: The more presumptions you have, the stupider you are. And the more you shed presumptions and learn to root them out, the smarter you are. I presumed I didn't need to react at all. I presumed that forgetting this obviously official piece of paper might be suitable behavior. They presumed I would act the way they wanted without express instructions.
How important are simple instructions? One of the great discoveries of direct mail is this: when you add in big bold letters to an outside envelope the words, Open This!, more people actually do!!! You might jump up your opening rate a soul-satisfying 10 or 20 percent. Never forget: inertia is a terrible thing to waste. Give. Clear. Instructions.