sleep deprivation

This morning on the way into work I stopped at the grocery store to pick up a cake and some other items for a coworker's birthday we'll be celebrating this afternoon. I didn't realize how tired I was until I picked out a cake (and then a different one), and the woman asked, "What would you like written on it?" I stared at her for a minute, thinking, and then replied, and spelled out the name...incorrectly. She looked at me and repeated back to me what I had just said. I looked at her for a second again, and shook my head, saying, "Sorry, I'm having problems. That's totally wrong" and respelled the name, which she repeated again, just to be sure. Sheesh.