Learn English – Word meaning: A slip of the tongue which suggests how you actually feel, often humorous

Wikipedia suggests two meanings for the physical action. The relevant one:

The ironic usage originates with the idea of suppressed mirth — biting one's tongue to prevent an outburst of laughter.

This matches with what I have always assumed was the answer: You are biting your tongue, cheek or lip to keep a straight face. The intent isn't so much to cause pain but to distract you from the joke or bluff. Another common behavior is to purse your lips when trying not to smile.

It's called a slip of the pen (more common), or slip of the keyboard (less common).

Either is fine, and will be understood. If you desire to be precise with your idioms, go ahead and use "slip of the keyboard". It's not in as many dictionaries/thesauruses, but people use it, and it makes enough sense that no one should misunderstand it.

I also agree with KeithS: typo would be fine in many cases of "slips of the keyboard", however the "slip" idioms usually indicate more of a mistake in the writer's mind (wrong choice of word) than a typing error, as with "slip of the tongue". "Typo" usually has to do with a small mistake involving a few letters, which the typist did not mean to include, whereas "slip of the pen" or "slip of the keyboard" indicates a mistake of greater scale, such as choosing the wrong word, which, at the time, the typist usually did mean to include, but later realizes his mistake.