The numbers game
Perhaps this is an issue for editors and proof readers but errors in numbers can be embarrassing. I don't want to sound patronising, but it is always worth checking.
A few errors I've noticed recently, two of which could have been avoided by looking at a ruler, are:
A prison exercise yard of 20 square feet. This would be about five feet by four - the author meant twenty feet square
A sniper bullet of 2mm calibre - i.e. not much bigger than the diameter of a cocktail stick
A bullet wound of 38mm diameter - nearly twice the size of the cannon shells used by Spitfires against the Luftwaffe during the later part of WW2. It should have been .38 (of an inch) - here metric and imperial calibres were mixed up
Finally, a reminder to check the correct abbreviation when using milligrams and micrograms. A milligram (abbreviated to mg) is one thousandth of a gram. A microgram (abbreviated to μg) is a thousandth of this (i.e. one millionth of a gram). They are easily confused!