Throwing an Octopus on the ice (at Joe Louis Areana) is good fun. Throwing Octopodes onto the ice will likely result in a penalty to the Red Wings…
Throwing Octopie on the ice would probably make a mess 
(get it? Octo-pie… you know… as in baked goods… oh nevermind…)
Well, it started out as ignorance - people, noting the ending, applied an analogy to other technical or semi-technical words ending in “-us” like “cactus” or “fungus” or “radius”. But I’d guess that I hear it at least as often as “octopuses” (which is the only plural that can be called ‘correct’ in English, as “octopodes” is attested but so rare as to be unrecognizable to most folks). It’s silly when you realize the word’s origins, but if you’re going to rail at all the folks who say “octopi” you’re going to have a hard time of it.
(I recommend instead you focus on people who say “indexes” and “appendixes” just because the Latinate plurals are so much fun.)
“What’s the plural for moose?”
“Moosen! I saw a flock of MOOSEN! There were many of ‘em. Many much moosen. Out in the woods…in the wood-es…in the woodsen."
Quoth Lumpy:
Of course, if we’re going to go down that route, then the whole structure of which the tentacles are a part is the “foot”. So I suppose that would make the tentacles themselves toes.
And this little piggy also went to the market, and this little piggy too, and also this one…
We must also consider the fact that in male octopuses one of the tentacles becomes modified to serve as a penis in inseminating the female. So male octopuses only have seven tentacle-legs/arms/whatever, plus one tentacle-penis.
“So a Babylon-Five Centauri male gets a job in Japanese tentacle porn and…”