WTF, Cupertino???

i have just stumbled upon a most peculiar Cupertino effect trap.  i am writing a paper for my phonology ii class on raddoppiamento in Italian.  it's a continuation of some previous work that i did last semester.  when writing my previous paper, i was running Mac OS X 10.4.  the system-wide spellchecker had no clue what this big, long, Italian word was, and offered up no possible corrections.  to avoid lots of little red underlines, i added the word to the dictionary and went merrily on my way.  since then i suffered a hard drive failure, which had two consequences: 1) the dictionary "forgot" that raddoppiamento is a word, as far as i'm concerned and 2) i've upgraded to Mac OS X 10.5.  no problem, i just have to re-teach the dictionary the word.

and that's when this happened:

whaaaaaaaaat?  where did that "d" come from??  i fired up dictionary.app (which presumably is the same dictionary resource used by the spellchecker) and, no surprise, raddoppiamdento is decidedly not in the dictionary either.  nor, as far as i can tell, is any substring that would fool the spelling suggestions algorithm (which i know for a fact does sometimes produce novel suggestions, especially when two words are accidentally conjoined) into thinking that raddoppiamdento was not only a possibility, but in fact preferable to raddoppiamento.

just for confirmation that i wasn't going crazy, i turned to Google, which seems to know which is the real word and which is the imposter (what the hell is going on, imposter just got flagged, despite the fact that dictionary.app gives both imposter and impostor):

um, yes?

if anyone has any hypothesis whatsoever as to what the great "improvement" Apple made to its spellchecker algorithm such that it produces these shenanigans, please leave a comment.