Kudo vs. kudos
Wrong: That’s deserving of a kudo.
Wrong: That’s deserving of many kudos.
Right: That’s deserving of kudos.
Kudos means praise or acclaim. Despite the “s” on the end, it’s singular, not plural. Therefore kudo as a singular form, and kudos used as a plural are both incorrect.
Mano a mano vs. man to man
Wrong: He’s going to take him on mano a mano.
Right: He’s going to take him on man-to-man.
Okay, this isn’t exactly an English phrase, but I see it misused so often in English writing that I felt the need to mention it. Presumably due to the spelling, many English speakers use the phrase as if it means “man to man.” In fact, mano a mano literally means “hand to hand” in Spanish, as in hand-to-hand combat. (I’ve also seen/heard it misused as mano y mano, or hand and hand, which makes even less sense.) There’s no point in spicing up your writing with foreign phrases if you’re going to use them incorrectly. If you mean one-on-one, head-to-head, or man-to-man, why not just say so?