At the climax of the Avengers, Tony Stark flies a nuclear missile through the wormhole created by the Tesseract. Shortly after he crosses the threshold of the wormhole and releases the missile, his suit's systems and even the ARC reactor appear to fail, and he passes out/goes into cardiac arrest. (This is an assumption on my part as the reactor no longer powering the electromagnet would allow for the shrapnel in his heart to continue moving further in.)
Throughout the major fight with the Chitauri he doesn't appear to take as significant damage as he does during his encounter on the Helicarrier, nor does the Mark VII armor appear to require significantly more power than the Mark VI.
So what would cause his systems to fail?
EDIT: I just rewatched the scene, and the suit appears to lose power just after crossing the threshold of the portal, as he's releasing the missile from his grasp, but Tony doesn't lose consciousness/go into cardiac arrest until just after the nuke makes impact.