Two legal terms that are often mistranslated into the Romance languages are "comparative law" and "derivative work."
Comparative law is obviously the study and comparison of the legal systems in different countries. In the Romance languages, its name would translate literally into English as "compared law," but we don't call it that. We call it "comparative law." Thus, we have:
French: droit comparé
Spanish: derecho comparado
Portuguese: direito comparado
Italian: diritto comparato
So note that if you translate, say, from Spanish to English, derecho comparado is not called "compared law" but comparative law and if you translate from English to Spanish, "comparative law" is not called "derecho comparativo" but derecho comparado.
The term derivative work works the same way in the Romance languages. In copyright law, a derivative work is one that substantially derives from another work that is copyrighted. Use of a derivative work constitutes copyright infringement if permission of the copyright owner is not obtained. In the Romance languages they say literally "derived work":
French: œuvre dérivée
Spanish: obra derivada
Portuguese: obra derivada
Italian: opera derivata
Therefore, œuvre dérivée is not "derived work" in English but derivative work, and "derivative work" is not œuvre dérivative in French but œuvre dérivée.
Let me know in the comments if you can think of any other legal terms that work like this (the adjective ends in IVE in English but the past participle is used in the Romance languages).