This is why mention of future planned activities is a bad idea. At least you didn't put it in its own space. It would help if you could share what the activity is, but I understand if you prefer to maintain anonymity. What I'd do, since I can't judge the relative impact of the activity's noncompletion, is mention the cancelation in a future update letter (no rush at this point). Ideally (and especially if the activity was important to fill a gap in your EC lineup), you will have found something to substitute for it that would have the same impact and mention in the letter that you have begun XXXX activity instead.
Thanks for the response. The future activity was actually research and development work in Africa. I've done a lot of work abroad and, at the end of one of my descriptions of this past work, I wrote a short sentence about how I would be continuing with my interests in development this summer through an internship in Africa. I only just found out that my funding has been approved to start at the end of June, which unfortunately does not work for my timetable (as it will overlap will school starting in Sept).
I actually have another opportunity lined up which I will be accepting -- global health research at a domestic institution. I am not exactly worried about the impact of this missing activity, since I already have a lot of research/development experiences on my application. Mostly, I'm worried that med schools might interpret my original description as misleading in some way. Would the best way be to simply include it in an update letter, or just not mention it at all?
Thank you again! I appreciate your help a lot.