Everything looked to be going to plan for the reigning world champions in their opening Group B game when Xabi Alonso gave Vicente del Bosque's men a 27th-minute lead from the penalty spot, converting after Stefan de Vrij tripped Diego Costa.
However, the Dutch drew level just before the break as Robin van Persie caught Casillas off his line with an audacious looping header and Arjen Robben added a second in the 53rd minute.
De Vrij atoned for his earlier error soon after the hour mark, nodding in from close range after Casillas misjudged the flight of a free-kick and the Real Madrid goalkeeper was at fault again for the Netherlands' fourth, as Van Persie pounced on his dreadful first touch to tap in.
Spain's obliteration was complete 10 minutes from time as the excellent Robben notched for a second time and captain Casillas accepted that he was at fault for the humiliating defeat.
He told Marca: "I wasn't up to scratch and I accept that.
"We weren't at the races, starting with myself. Now all we can do is focus on the next game.
"I accept the blame. We have to think about the next match. Our heads dropped when they scored the third goal.
"Now we have to be quiet, reflect and think about the next match against Chile, which we will approach like a final.
"We owe the fans an apology, myself first and foremost. We have to accept all the criticism that will come our way and forget what happened as soon as possible.
"We're mature enough to take the criticism. We have to face the music."
