This article reports on an experiment in which two components of translator students’ professional self-understanding were challenged, viz. their doubts on the relevance of theoretical knowledge as part of their professional competence as well as the strong belief in their L1 competence. The experiment draws on Toury’s “law of interference” and the analysis of students’ translations is based on Tirkkonen-Condit’s hypothesis that unique, TL-specific elements may be underrepresented in translations. Students’ translations of short English and German source texts are consistent with this hypothesis: the experiment reveals that even a source text that seems to present no translation difficulties in surface structure is still a powerful constraint in translation and produces language patterns which are alien to or at least deviant from non-translated target language usage as revealed in this experiment by a small-scale cloze test.