First off let’s state what most people will recognize to be the obvious. We cannot 100% exclude the possibility of error in any human activity. This includes the activities of professional human translators.
That said, we can also say that professional human translators are responsible for preparing the highest quality translations commercially available. They consistently produce superior results compared to any current machine translation technology.
Quality assurance measures associated with human translation services such as post-translation proofreading by a second linguist contribute to achieving high quality in translations. In some translation projects multiple levels of QA reviews are conducted. So by the time a translation client receives a prepared translation they can be confident of the translation’s high quality.
So while outright errors committed by professional translators are infrequent, translation clients can sometimes request changes to translations for reasons other than errors. A couple reasons translation clients request changes to delivered translations are:
They have a preference for a particular translation such as a keyword or phrase that’s commonly used in their business or line of work. This can be a good and proper reason for a translation client to request a change. Even better would be when a client provides a terminology list of preferred translations prior to the start of a project.
Someone in their organization has a preference for a particular word usage or translation. This is not always a good reason to change a professionally prepared translation since customers buy and benefit from the language judgment of professional linguists. Substituting the language preferences of a lay reviewer, overriding the judgment of a professional linguist, can defeat the purpose of hiring a professional in the first place.