Back-translation refers to the process of translating a target text back into the source language in order to check for correctness and appropriateness of expression.
It is a useful tool to aid translators who are not native speakers of the target language. However, back translation does not always indicate an error in translation from the source-to-target result, as the problem may be with the back translation itself.
Also, idiomatic phrases will always trip up translations; for example, "out of signt, out of mind" --> invisible idiot