An error I encounter two or three times a week is the use of "diffuse" when the sentence calls for "defuse." In every case, the sentence refers to the need to defuse a situation, though it is "diffuse" that is used.
To "defuse" literally means to remove the fuse from something--e.g., a mine or a bomb. Metaphorically, to defuse a situation means to render it harmless (or at least less harmful), to prevent it from exploding.
I believe the error occurs not just because the two words sound similar, but also because the word "diffuse" means to scatter, to spread thinly, or to break up and distribute something. The people who make this error might well be thinking that by "diffusing" a situation, one is breaking it up and thus reducing the tension in it.
But that is not what the idiom actually means. It means to remove the fuse from an explosive situation, and the proper word is "defuse."