There was an incident in an earlier competition that had people crying foul against Big Brother. It was the the week they were trying to evict Allison/Ryan. The veto comp where the girls were dressed as cupid. The boys were attached to them by a rope and they had to run back and forth bringing pieces to the girls to complete a puzzle.
Something they didn't air, Adam's rope got tangled and they had to stop the game to fix his ropes. Before the game was stopped, Natalie had dropped one of her pieces behind the scenery. There was no way they could finish the puzzle without that last piece because Matt didn't see it happen and Natalie didn't want to tell him. Well, when they stopped the comp for Adam's technichal difficulties, a crew member went and picked up the errant piece and gave it to Matt. When the comp resumed, Matt and Natalie ended up winning the POV. If they hadn't stopped the comp for tech difficulties, Allison and Ryan would probably have won. Because they were almost neck and neck with Natalie and Matt. And if Matt had to go looking for that missing piece, Ryan would have passed him by and they would have won POV and taken themselves off the block.
Now I don't think BB rigged that comp to obtain a certain outcome. But, they definitely changed the outcome by their actions.