For example, if you try to translate "condescending" to Spanish it will give you "condescendiente", but that means "to tend to adapt to the likes and wishes of others", which is completely different from the English definition of "to display a patronizing, superior attitude".
Translate "fallacy" and it will give you "falacia", which is kind of similar, but different. In English it means "incorrectness of reasoning", but in Spanish it means "false argument to trick or cause error", meaning that the Spanish word implies malicious intent.
Translate "patronize" and it will give you "patrocinar", which means "to sponsor". "patrocinar" can't be used in the sense of "to treat in a condescending manner".
It's pretty hard to catch these kinds of things until you're really experienced with both languages.