In this thesis I aimed to demonstrate common translation mistakes of two statistical Machine Translation systems, Google Translate and Webfordítás and compare them through a case study. I carried out a research in human evaluation of Machine Translation, in which I compared two statistical Machine Translation systems, Google Translate and Webfordítás. The 6th chapter describes the details of my questionnaire, such as the Machine Translation systems I chose to compare, the subjects, the number and genre of sentences translated and the instructions that the subjects were given. The 7th chapter looks at the evaluation results of this questionnaire sentence by sentence, also by genres, by adequacy, by fluency and finally by Machine Translation systems, the 8th chapter was the qualitative analysis of the answers of the subjects.