Most Human-Computer Interaction (HCI) researchers are accustomed to the process of formal ethics review for their evaluation or field trial protocol. Although this process varies by country, the underlying principles are universal. While this process is often a formality, for field research or lab-based studies with vulnerable users, formal ethics requirements can be challenging to navigate -- a common occurrence in the social sciences; yet, in many cases, foreign to HCI researchers. Nevertheless, with the increase in new areas of research such as mobile technologies for marginalized populations or assistive technologies, this is a current reality. In this paper we present our experiences and challenges in conducting several studies that evaluate interactive systems in difficult settings, from the perspective of the ethics process. Based on these, we draft recommendations for mitigating the effect of such challenges to the ethical conduct of research. We then issue a call for interaction researchers, together with policy makers, to refine existing ethics guidelines and protocols in order to more accurately capture the particularities of such field-based evaluations, qualitative studies, challenging lab-based evaluations, and ethnographic observations.