@noelle This happened to me once. A customer called to say that the program I wrote showed wrong days of the week. The program had been running for years in a couple of dozen places, and it had some glitches, but never this. I spent a few hours reviewing the code, then I went to the customer's office and sat at his desk. There was this little desk calendar by the computer. The days of the week were all wrong for that month. A fucking printing error at the calendar factory that cost me a few hours of work and frayed nerves (I take such things seriously).