Quite probably one in one of my very own programs. I am fascinated by Sun and Moon and made some Visual Basic programs that track the sun and moon across the sky, either as little icons that move on top (Z dimension) of your other open windows, or as a picture, or as digits. A parameter file lets you put in things like the moon's phase, your height (for shadows), earth's tilt (can have fun with this one), and up to 96 latitude / longitude pairs for examination. Anyway I was on me hols once and opened up my little sun bars (it's my own desktop toy, you see). Although there was no error, they were all f***ed up with the calculations clearly wrong and the sun / moon not where they should be for the lat and lon I entered. I puzzled over this for ages. In the end the "bug" was very simple!! I had used my (Dutch) wife's laptop, which used a comma as a decimal point rather than a full stop!