I sometimes see or hear these verbs used interchangeably.

Problem:
The verbs “defuse” and “diffuse” are not synonyms.

Explanation:
The verb “defuse” — spelled D-E-F-U-S-E — was coined in the early 1940s and means to remove the fuse from, where the “de” prefix in this verb indicates negation and the “fuse” suffix refers to a cord of readily combustible material and comes from the Italian word “fuso”, which literally means spindle.

Unless you are a terrorist or sadist, you want the police to defuse a bomb.

The verb “diffuse” — spelled D-I-F-F-U-S-E — dates back to the late 1300s and means to spread or disseminate, where the “dif” prefix means apart or away and the “fuse” suffix comes from a form of the Latin word “fundere”, which literally means to melt.

Once the police have defused a bomb, they often will diffuse a message to the public about their success.

I believe that the oral/auditory confusion comes from mispronunciations of these two verbs.

I believe that the written confusion comes from ignorance about the roots of these two verbs and from misspellings originating from that ignorance or the mispronunciations.

Solution:
Remember that the “de” in “defuse” indicates negation, and remember that the “dif” in “diffuse” means apart or away.