The documentation for get_exceptions says "As a class method with no parameters it returns all the default exceptions as an anonymous hash of 'exception' => 'replace with' pairs" - which seems to suggest there might be some! But I also searched the code and couldn't find any. It isn't easy to invent a list of words which the stem algorithm stems incorrectly (such as this stemming to thi) so I hoped someone might have done the work to come up with a list of common words. Otherwise the only way I can think of doing it is to stem a large quantity of text and examine the results - rather laborious even if sorted by frequency.