A reader sent compliments about a feature on British troops sweeping for bombs while on patrol in Afghanistan ... but not about the repeated misspelling of a key word: "The Coldstream Guards featured in your article are surely trying to defuse IEDs, not spread them around more by 'diffusing' them."
Mr. Rewrite sees defuse/diffuse confusion quite often. A quick check of Google News for diffuse bombs and diffusing bombs, for example, turns up articles by news organizations that should know better.
Defuse means rendering something safe, be it a situation or a bomb. Diffuse means dispersing or evenly spreading something from a central location or source.
Bryan A. Garner (cue angelic chorus) suggests that the notion of light being diffused, or softened, could explain why some people confuse diffuse for defuse. Mr. Rewrite thinks it's just inattention with words that sound alike, something he suspects stems in may cases from a lack of reading.
If Mr. Rewrite doesn't stop blogging and tweeting so much he'll have to defuse tension with Mrs. Rewrite over access to the computer.