Trade in genetically modified products is a longstanding and contentious issue in agricultural trade. One issue has not, as yet, received much attention. This is the mingling of unapproved GM products with conventional products. This issue is likely to gain more prominence in the future as new GM product development accelerates. Until recently, problems with mingling were largely one-off events. Recently, however, an ongoing case of mingling has arisen – the case of Canadian GM flax in the EU. The case led to an embargo of imports from Canada and subsequently the bilateral negotiation of a protocol to allow exports to resume. The case raises a number of important issues pertaining to the objective of zero tolerance policies for GM products, the operationalisation of zero tolerance, the role of the testing industry, the design of testing regimes and the risks associated with the absence of transparency and/or international standardisation. It is concluded that mingling is a topic that is deserving of multilateral attention.