There have been quite a few referee controversies already at the World Cup when it comes to offside calls. In Mexico's game against Cameroon, the refs made two bogus offside calls costing the Mexicans two goals.
In the 71st minute the favored Swiss had a chance to go up 2-1 when the refs disallowed a goal for offside in their game against Ecuador.
When Josip Drmic was fed the ball by his teammate, he was clearly onside:
The Swiss were able to come back and win the game 2-1 with just 20 seconds left in stoppage time, but sometimes goal differential becomes important as teams try and make it out of the group stage.