Versus vs. Verses
As in the case of "affect" vs. "effect," the usage problem has two causes. The words look similar, differing in only one character in the same place in each word. Also, the sound we use to pronounce those differing characters tend to be the same sound. This causes our minds to confuse the Latin word for "toward" (spelled versus!!) with the Latin word for "row" or "line" (also spelled versus!!).