slip of the pen

I’ve seen UK Uncut make this mistake on Twitter before now so it’s not just a slip of the pen or a sudden seizure or brain spasm.

Is not Bacon here a slip of the pen or press?

When we were young, Hal, the means of acquiring knowledge generally, were limited by circumstances; but who that wishes, lacks them at present?—Well, my daughter"—— "Yes, pa, I see,—of course it was a mere slip of the pen"—— "A slip of the pen!" retorted the father, "and is that a sufficient excuse?

This fact in itself must prove to us that he considers the error, the slip of the tongue for instance, as meaningful; for he creates it on purpose, and it is not a case of the poet committing an accidental slip of the pen and then letting his pen-slip stand as a tongue-slip of his character.

This looks as if these persons knew that they had made a slip of the pen while writing the letter.