An MT engine can never get everything hundred per cent right simply because not all source sentences are absolutely unambiguous. In every language there are ambiguous or equivocal terms and many sentences require certain additional knowledge (of the context) contained for example in the previous paragraph or elsewhere in the preceding text or simply resulting from some general knowledge.
There can be swapped object and subject, active and passive voice, some mistaken understanding of the structure of the sentence or misunderstood word meaning. Many sentences are hard to understand and translate even for an experienced human translator who has a good command of both the source and target language because the source sentence does not necessarily contain all information needed for correct and precise translation. Moreover, most sentences can be correctly translated in multiple ways but some translations are just less suitable in a given context or in terms of their style.
A machine translator is not meant to entirely replace a certified human translator. A machine translator is a tool performing certain tasks for which it is designed and trained. It can for example make work easier for a human translator (by saving his/her time) or enable a human unfamiliar with the source language to find out certain information from the text. A great tool for these purposes is an integrated electronic dictionary which can be used by the user to verify the correctness of some important sections of the translation.
A user who is not familiar with the target language is able to use a machine translator and the dictionary to quickly obtain certain relevant information from the text with a certain degree of certitude without having to assign it to a human translator and waiting for the translation.