Sometimes you may find that your mouth gets slightly ahead of your brain and a buffer underrun occurs. This causes data corruption which, more often than not, results in at least one conversational typo.
The frequency of conversational typos is often increased by the typoer being under pressure or nervous. This can usually be attributed to their processing speed being slowed as the speech processing shares braintime with thoughts of being looked at by a large number of people.
One of the most common forms of conversational typo is when the mouth speaks a partially-formed idea before it has been structured correctly, resulting in a broken sentence with incorrect grammar and structure. An example of this might be "I was quite fast... -ly running." instead of "I was running quite fast."
With many people, the occurance of one conversational typo is often followed by more for the same reason as that of the increase when under pressure. Many people attempt to excuse conversational typos by making deliberate gibberish noises. This can be amusing in the right company, but can also create an undesired image among an audience of the wrong type.
The term 'conversational typo' seems to have been coined to refer to this phenomenon by people who frequent texual communication systems such as IRC. The association with a conventional typo is a weak one, as a normal typo refers to an accidental misorder of keypresses. However, both conversational and conventional typos result in not saying what you meant, so the name is a fair one.
This term is also used to describe accidentally mispronouncing a word or phrase, perhaps because the toungue doesn't articulate quite as intended. This does often lead to some conversation typos of the kind I described above, too.
Other words to describe this type of mistake include "moutho" and "braino". These two are a little different though as they describe only one of the two kinds of conversational typo I described: the former blames the mouth, and thus refers to the latter type of conversational typo. The term "braino", on the other hand, curses the brain for not delivering coherant sentences fast enough to be rendered into speech and thus refers to the kind of conversational typo I described first. A third alternative is "speako", which I take to be equivalent to "conversational typo" in that it simply refers to a speaking mistake (as "typo" refers to a typing mistake) rather than specifying the nature of the mistake.
Since all of these terms are slang, their meaning is of course ultimately flexible. I doubt anyone will complain if they are used interchangably.