Box Item Duplicating
From SourceRuns
Box Item Duplicating is a glitch that spawns the item inside of a box twice when the box gets destroyed. To trigger the glitch, an explosive box must be nearby.
Explanation and How-to
When you destroy both a crate with an item inside and an explosive crate in one shot (using a shotgun), the crate with the item will break twice - once from the shotgun bullet and again from the explosion. This will make the item spawn twice. Since the shotgun fires multiple bullets (with a decent spread), you may want to damage the crate a bit (without breaking it) first, and then shoot inbetween the first crate and the explosive crate, to ensure that both crates will break.