The paper reports on the creation, annotation and curation of the MT@BZ corpus, a bilingual (Italian–South Tyrolean German) corpus of machine-translated legal texts from the officially multilingual Province of Bolzano, Italy. It is the first human error-annotated corpus (using an adapted SCATE taxonomy) of machine-translated legal texts in this language combination that includes a lesser-used standard variety. The data of the project will be made available on GitHub and another repository. The output of the customized engine achieved notably better BLEU, TER and chrF2 scores than the baseline. Over 50% of the segments needed no human revision due to customization. The most frequent error categories were mistranslations and bilingual (legal) terminology errors. Our contribution brings fine-grained insights to Machine translation evaluation research, as it concerns a less common language combination, a lesser-used language variety and a societally relevant specialized domain. Such results are necessary to implement and inform the use of MT in institutional contexts of smaller language communities.