Ken Mac Dougall bit into the sandwich his wife had packed him for lunch and noticed something odd—a Post-it note tucked between the ham and the cheese. He pulled it out of his mouth, smoothed the crinkles and read what his wife had written: "Be in aisle 10 of Home Depot tonight at 6 p.m."

Mr. Mac Dougall was renovating the couple's Oak Ridge, N.J., kitchen, and his wife had been urging him to pick out the floor tiles. He felt he had plenty of time to do this task. She felt unheard.

"I...