(Although technically not accurate, false friends are often referred to as false cognates. Presumably, that would make fickle friends known as partial cognates.)
To take an extreme example of a fickle friend, one so extreme it is on the list of false friends, look at molestar, which is related to the English verb "to molest." In English, the verb can mean "to bother," which is its Spanish meaning, as in the sentence "they continued on their journey unmolested." But far more often, almost always, the English word has a sexual connotation that is absent in Spanish.
Many of the words on the following list are something like that, in that they have a meaning similar to an English one but often mean something different. Translating them as the English cognates may make sense some of the time but frequently it won't.
- Admirar: It can mean "to admire." But it frequently means "to surprise" or "to astonish."
- Acción: It is usually synonymous with "action" in its various meanings. But to a stock broker it can also mean a "share," and to an artist it can be "posture" or "pose."
- Afección: Once in a while, this word does refer to a fondness toward somebody or something. But far more commonly it refers to a disease or some other sort of medical condition. Better words for "affection" are another cognate: afecto, and a separate word, cariño.
- Aparente: It can mean the same as the English "apparent." However, the Spanish usually carries a strong implication that things aren't what they appear to be. Thus, aparentemente fue a la tienda would usually be understood not as "he apparently went to the store" but as "it appeared like he had gone to the store but he didn't."
- Agonía: Nobody wants to be in agony, but the Spanish agonía is much worse, usually suggesting that someone is in the final stages of death.
- Americano: The understanding of this word varies from place to place. If you're from the United States, it's safest to say soy de los Estados Unidos.
- Aplicar: Yes, this word does mean "apply," as in applying an ointment or a theory. But if you're applying for a job, use solicitar (although there is some regional usage of aplicar). Similarly, an application for a job or something else you would apply for is a solicitud.
- Apología: The Spanish word doesn't have anything to do with saying you're sorry. But it is synonymous with the English word "apology" only when it means "a defense," as in a defense of the faith. An apology in the usual sense of the word is excusa or disculpa.
- Arena: In sports, arena can refer to an arena. But it is more commonly used as the word for "sand."
- Argumento: This word and its verb form, argumentar, refer to the type of argument a lawyer might make. It can also refer to the theme of a book, play or similar work. On the other hand, a quarrel could be a discusión or disputa.