All right, so, we as World Cup watchers should probably take the following into account: that linesmen making an offsides decision are trying to follow multiple players and the ball at once, making it the hardest call in the sport; that in the jumble of bodies on a corner kick situation, it can get even more difficult; that they are human; etc.
But yeah, even taking all of that into account, it was not a good first half for the poor linesman tasked with officiating the Mexico-Cameroon match, who disallowed not one but two perfectly good goals for El Tri's Gio Dos Santos.
Goal the first, via SBNation:
Goal the second is more complicated -- Dos Santos did slip into an offside position on Mexico's corner kick, and the ball was deflected to him. But it was deflected by a Cameroon player, as this Kyle Bonagura screencap shows ...
Cameroon player heads it to Mexico player, who scores. Offsides, Mexico. pic.twitter.com/Xk0Krw0JMh— Kyle Bonagura (@BonaguraESPN) June 13, 2014
...and a player can't be ruled offsides if the ball is played to him by an opponent.
Mexico still doesn't have to be tied 0-0, since they could have taken one of their multiple other chances. Most notably: a free kick destined to give Hector Moreno a free header from three yards that was deflected away ... by his own teammate.
But there's no way to say "they have nobody to blame but themselves" and mean it. They absolutely have the linesman to blame.