Prone/supine: The bad guy stabs the victim in the chest. The victim falls backward and lies prone on the floor. No, actually he’s supine. Prone means face downward. Supine is face up.
Discreet/discrete: If your heroine is acting discreetly, she’s not telling everyone what’s she’s doing. Discrete, means individual and separate.
Quite/quiet: A common typo that isn’t fixed by using spell check.
Form/from: Another common typo that isn’t fixed by using spell check.
Junction/juncture: A junction is where things join. Like a road junction. Juncture is an event that occurs at a specific time. An astounding number of romance novelists don’t know this difference.
Peak/peek/pique: Peak is the top of a hill. Peek means to look at. Pique is when something has upset you. I am frequently piqued when authors misuse peek and peak.
When in doubt, remember that the dictionary is your friend.