Mad-Eye Moody, as we learn at the end of Goblet of Fire, has been conspiring over the course of the year to help Harry win the Triwizard Tournament. During the Third Task this involved him sabotaging the other contestants and clearing Harry's path of potential dangers.
"You had an easier time of it than you should have done in that maze tonight, of course," said Moody. "That was because I was patrolling around it, able to see through the outer hedges, able to curse many obstacles out of your way. I Stunned Fleur Delacour as she passed. I put the Imperius Curse on Krum, so that he would finish Diggory, and leave your path to the Cup clear."
(Goblet of Fire, Chapter 35, Veritaserum)
The very nature of Moody's work is that it should be undertaken quietly and without anyone noticing - "so that [his] hand would not be detectable in [Harry's] success". As one of the four helpers patrolling the maze Moody was in a good position to act and his magical eye certainly gave him an unique vantage point. Nevertheless, any intervention would surely have been extremely obvious.
Granted, the Triwizard Tournament was an atrocious spectator sport, and the Third Task was scarcely better than the Second Task in this regard. Still, the Task took place in the Quidditch stadium, which was presumably routinely equipped for 360-degree panoramic viewing. The crowd may not be able to see inside the maze but they'd have had a good view of the outside of the maze. A stunning spell is associated with a burst of red light in Harry Potter and Moody's spell may well have burnt a hole in the maze. Other spells would have been equally visible. Regardless, the crowd would have seen Moody pointing his wand into the maze, casting spells and self-evidently interfering with the task.
How did Moody get away with manipulating the task so much? Shouldn't it have blown his cover?