The army wasn't exaggerating or just playing a prank either. Karl Urban says that sometimes when you looked down in the middle of a fight scene, you'd see something like a bomb's fins coming out of the ground. As running around and jumping up and down isn't recommended when you're on a minefield, the army would be coming in every day and cleaning up the area, removing bomb after bomb like a bunch of Jeremy Renners. At one point, some of the area was sealed off with tape, which caused Viggo Mortensen to ask if everything outside was safe -- turned out the answer was no, the area outside the tape just had fewer bombs.
In the shot where Aragorn and the others ride toward the Black Gate, Peter Jackson was particularly anxious about mines. He remembered the army outlining the zone that was cleared, and he thought that the actors might have gone a little outside that zone -- so he and the crew were just dreading the possibility of them riding onto a land mine. Luckily, it turned okay, and we didn't end up with a Fellowship Of The In Memoriam at the Oscars.
However, Jackson's concern seemed to be a one time deal as he was otherwise almost giggling with trollish delight about the fact that the movie was being shot in a place where there was a chance that you could get blown up if you took the wrong step. "Typical Pete Jackson," said Dominic Monaghan, which makes us wonder how many ghosts in The Frighteners were just spirits of crew members from his prior films. According to Viggo Mortensen, the whole thing layered a bit of tension and chaos onto what was already a very chaotic shoot.