When I am reading manuscripts or editing or examining, one of my favourite typos is "periscopes" for "pericopes". The word pericope is a favourite among Biblical scholars, for a unit of text, a passage, especially in the Gospels, especially in contexts connected with form-criticism. Even as I type this entry, "pericopes" is getting underlined in red as a spelling error, while "periscopes" is not. Since the advent of spell-checkers over the last generation or so, I suspect that this typo has become much more common. The eye misses the extra "s" and it is not, of course, picked up by the spell-checker. The other day I glanced at my hand-out in class and saw the word "periscopes" there where it should have been pericopes, so I had fallen prey to the same error. Perhaps this is a reason to stick to the slightly affected plural "pericopae"?