Certain words are, for whatever psychological reasons, tricky for certain people to read or say. In some cases it's just a minor impediment, perhaps a juvenile quirk that got locked down too early in the language-learning process. Pisghetti instead of spaghetti is a classic example that many have noted. And I still have to consciously push the "f" sound into sphere, lest it come out spear.
Then there are terms that are too foreign, too medical, or too closely connected to delicate issues for flawless mental parsing. Robert Anton Wilson's Illuminati über-conspiratorial yarn hypothesizes that folks may be socialized not to see particular strings of characters, e.g., the word "fnord" <--- (this space intentionally left blank). In the movie Beetlejuice, for instance, the Handbook for the Recently Deceased explains that ghosts aren't actually invisible; most of the Living just choose not to see them. Maybe it's connected to the Edgar Allan Poe "Purloined Letter" technique of hiding something in plain sight.
In the opposite direction, a mind may insert additional letters during preconscious phases of data processing:
Wisdom suggests that one not spend too much energy attempting to correct a victim of such psycholinguistic glitches ...