What is the silliest mistake you have ever committed?

It was about 22-23 years ago. I was developing a simple game engine in watcom c and dos extender (does anyone remember this technology?). I used a pregenerated sin table, but it looked very silly in the source, because it wasn't aligned, so I decided to use '0' characters to pad the numbers. Later I tested the program (and have forgot about the formatting long ago), everything was messed up. It took a few hours debugging to realize that 0something numbers mean octal in C...
I do commit silly mistakes but I will remember this forever.

Comment a custom log function. Without that log function, the application didn't print anything.

And well, from that moment I realized that I need to take notes about my progress more often.