In spite of recent successes in improving Machine Translation (MT) quality overall, MT engines require a large amount of resources, which leads to markedly lower quality for lesser-resourced languages. This study explores the case of translation from English into Igbo, a very low resource language spoken by about 45 million speakers. With the aim of improving MT quality in this scenario, we investigate methods for guided detection of critical/harmful MT errors, more specifically those caused by non-compositional multi-word expressions and polysemy. We have designed diagnostic tests for these cases and applied them to collections of medical texts from CDC, Cochrane, NCDC, NHS and WHO.