Leak and Leek are homophones and easily confused words. The spell-check application in most word processing software wouldn’t catch a mistake of these two words. Spell-check aims to tell you what isn’t a word, and what words aren’t spelled like the ones in its own dictionary. If it’s a word, and it’s spelled correctly, spell-check keeps right on looking.
Leak is a noun, meaning a place where water or other liquid is escaping where it should not be, for example: the faucet in a sink, a burst plumbing pipe, a fire hydrant, or water spigot, or a plastic bottle with a hole in it or whose lid is not properly fastened.
Leek is a noun meaning a type of onion. Leeks are characterized by large blade leaves that extend from their bottom bulb. Typically, only the bottom third of the plant is what’s consumed. [ TRIVIA: Leeks are a national symbol of Wales.]
The following story uses both words correctly:
Luke had planned to make leek soup for dinner, but a milk jug in his refrigerator had a leak in it. It made a huge smelly mess, and managed to rot his entire vegetable drawer.