@kfingerman: Wikipedia is commenting on current usage, in which people who are trying to sound educated use the incorrect plural octopi.
In English, you can correctly form the plural of loan words either by treating them as English (thus, octopus, octopuses; index, indexes) or by forming the plural in their native language (octopus, octopodes; index, indices). Either of these is correct, although which one is preferable depends on the audience you’re writing for.
If you misread the word origin and form the plural incorrectly (such as octopus, octopi on the model of the -us, -i declension in Latin, when octopus comes from Greek), or when you don’t even do it correctly in terms of the original language (such as virus, which is in the -us, -us declension in Latin, and even if it were in the -us, -i declension, the plural would be viri, not virii) you just look like an idiot with delusions of sophistication.
Someone who has more time than I do ought to correct Wikipedia. The virtue of Wikipedia is not that it is always correct (because any ass can see that it is not) but that when it is incorrect, it can be corrected.