Uncertainty and complexity are two factors that make project planning a challenging task. Companies operating in a multiple project Engineer-to-Order environment face an additional dimension of difficulties because of interactions between projects. The changing environment and frequency of unforeseen events increases the difficulty of planning and re-planning and requires flexibility. This study explores how project planning is conducted and what the associated challenges are in a multiple project ETO context. To that end, a case study of a company operating in such a context was investigated using a qualitative approach. In so doing, we shed light on the challenges and provide some insight to mitigate the challenges. The findings suggest that there are challenges related to resource allocation, prioritization, communication and progress updates, and keeping the plan up to date and valid. Practically, the main takeaways are that synergies may be found when leveraging workflow reliability and flexibility as it comes down to deciding when the respective approach is suitable to mitigate the challenges found in the study. In addition, we suggest that prioritization should be integrated into the planning processes to create a better consensus within and among projects. Lastly, we also suggest an additional precondition for releasing work to production.