The peninsular peninsula
There is an interesting group of words that changes their spelling according to usage (in British/Aus English at least). For example, practice (noun) versus practise (verb), and licence (noun) versus license (verb). Less well known to me is peninsula (noun) versus peninsular (adjective). People often misspell the former because of its similarity to the easy-to-spell insular. Once you note that both insular and peninsular are the adjective forms, it becomes easy to remember the odd one out.
As you might expect, these words have the same origin. Insula is Latin for island, thus an insular nature is isolated, like from an island. Peninsular has the pene Latin prefix, meaning almost. Hence, it's a piece of land that's almost an island.