Obvious ... But Wrong

• Sano: Someone who is sane is en su juicio or "in his right mind."
• Sensible: Usually means sensitive or capable of feeling. A sensible person or idea can be referred to as sensato or razonable.
• Sensiblemente: Usually means "perceptibly" or "appreciably," sometimes "painfully." A good synonym for "sensibly" is sesudamente.
• Sopa: Soup, not soap. Soap is jabón.
• Suceso: Merely an event or happening, sometimes a crime. A success is un éxito.
• Tuna: Order this at a desert restaurant and you'll get edible cactus. A tuna is also a college musical glee club. The fish is atún.

A final note: Especially in the United States, Spanish doesn't exist in a vacuum. In the United States, you may hear some speakers, especially those who frequently speak Spanglish, use some of these false cognates when speaking Spanish. A few of these usages may be creeping into the language elsewhere, although they would still be considered substandard.