Shortly thereafter, the Eleventh Doctor arrives in Cwmtaff with Amy Pond and Rory Williams, having promised them a vacation in Rio de Janeiro, missing by several thousand miles; the TARDIS has brought them to the wrong place yet again, much to the annoyance of the companions. As they look across the valley, Amy spots two figures waving at them from the opposite hillside. It is Amy and Rory from ten years in their future, coming back to revisit past glories. Though she finds this "interaction" with her future self thrilling, Amy still wants to go to their intended destination.
The revisiting of "past glories" must have happened in between the events of two episodes. You mustn't forget that sometimes there can be a big gap in between episodes, of which we can only speculate what happened. Also, "The Hungry Earth" is the eighth episode of the fifth series, and "The Angels Take Manhattan" (in which Amy and Rory get sent back in time) is the fifth episode of the seventh series. A lot of time in between, 'ey?
Some people think that it was actually their future selves warning them for something, and not waving at them.
Well, whatever the case, I am sure it has nothing to do with and it doesn't interfere with the unlucky events in "The Angels Take Manhattan".
Besides, their future selves were standing quite far away from their present-day selves, so the distance wouldn't have been enough to create a fixed point in time. Also, they didn't talk to each other, while Amy did say farewell to the Doctor in The Angels Take Manhattan, thus creating a fixed point.