Adventitious presence of genetically modified crops (AGMP) in their non-GM counterparts is an important problem in the EU, where a policy of coexistence between the two is being sought. The economic impacts of AGMP could be severe and could prevent the practical application of GM coexistence measures. To date, research has concentrated on the contribution of pollen gene flow as the major source of AGMP, while other sources have not been investigated. We have examined the potential for AGMP from the use of shared farm equipment and transport, which has previously been assumed to have a low contribution. Oilseed rape (OSR) was examined in a typical UK production regime including tillage, sowing, spraying, harvesting and grain drying. At each stage in the process, OSR in the machinery was measured and its potential for AGMP calculated. In sowing and grain drying, mustard grain was used as a proxy for GM grain and was measured during the processes by real-time PCR quantification to give estimates of GM dilution rates during the processes. The effects of cleaning and other mitigation methods were examined. Total potential AGMP was estimated at 1.47% when no mitigation was performed, and 0.08% when machinery was cleaned. The best measures for avoiding this type of AGMP are presented in the context of the specific UK agriculture examined in this study.