Snaffled a protractor off my colleague's desk. Took a few measurements. Put said protractor back. Performed a few calculations. Needed to take another measurement. Picked up his protractor again. Noticed that it's in gradians.
(For the uninitiated, gradians are a unit of measuring angles, apparently used occasionally in Germany and Scandinavia. There are 400 gradians in a full circle, so, for measurements of around 15-20 degrees, it's not immediately apparent that there is a small but significant difference between the gradian and degree measurements.)