According to open future views, the proposition that in 2015 a fair and indeterministic coin lands heads has some probability but is not true. However, that proposition is apt to become true in 2015. So the probability of the proposition isn't the same as the probability of the proposition being true, since it's certainly not true now, but might well become true in 2015.
So far so good (or bad). Suppose God promises you that from 2015 onward, every year, a fair and indeterministic coin will be tossed. Now let Q be the proposition that there are infinitely many years after 2014 during each of which a fair and indeterministic coin lands heads [I screwed up in the original formulation of Q and wrote "that every year from 2015 onward, ad infinitum, a fair and indeterministic coin lands heads"; Alan Rhoda's response targets my screwup; see my response to him below]. Now note that on open future views Q can never possibly become true. For on any date, the proposition requires for its truth that there will be infinitely many fair and indeterministic heads results still past that date, and on open future views a proposition that requires an undetermined future event won't be true.
So, open future views have to say that it's impossible for Q to ever be true. But a proposition such that it's impossible for it ever to be true should get probability zero. But the probability that of the infinitely many coin tosses, infinitely many will be heads is 1 according to classical probability theory. So open future views should be rejected.
Here's another argument in the same vein. Suppose I know I will have an eternal afterlife, and I promise you that I will freely pray for you every day, ad infinitum, starting November 1, 2014. On open future views, the object of my promise is a proposition that can never be true. But it's clearly a bad thing to promise something that can never be true. Yet what I promised wasn't a bad thing to promise. So open future views are false.
One might even have the direct intuition that one could keep the promise. That intuition is incompatible with open future views.