Ack. I've slept in on Friday, thinking it was Saturday, and I've gone to work on Saturday a couple of times, thinking it was Friday. I've sat at at my desk working for an hour before I noticed how quiet the office was.
The worst was when I was a mechanic, years ago. I noticed the service department hadn't been opened, so I unlocked, turned on the lights, fired up the service computers, made coffee, etc. I wrote up a few drop-ins, and wondered why it was so slow, and why the service advisor hadn't shown...
I ran out and locked the gate, but had no choice but to work on 3 or 4 cars on my day off.