One amusing usage error is when people use tenant when they mean tenet. A tenant, of course, is someone renting a property, while a tenet is “a principle, belief, or doctrine generally held to be true.”
Another pair of commonly confused words is complementary and complimentary.
There are so many words that are spelled similarly to each other and therefore frequently get mixed up in writing. One of the most common pairs of these words is effect and affect. And while most writers usually know the difference between them, sometimes these words still get confused.