Why does the porter stemming algorithm online at
http://text-processing.com/demo/stem/
stem fried to fri and not fry?
I can't recall any words ending with ied past tense in English that have a nominative form ending with i.
Is this a bug?
A stem as returned by Porter Stemmer is not necessarily the base form of a verb, or a valid word at all. If you're looking for that, you need to look for a lemmatizer instead.