After the Politburo deposed Erich Honecker in an effort to reduce the repressive image of the regime, Günter Schabowski became East Germany's press spokesman.
On Thursday, November 9, 1989, the Politburo decided to ease pressure by beginning to allow East Germans to travel in a controlled fashion to the West, after applying and receiving permission from the regime, to take effect after border guards had been retrained. Schabowski was given a note about the decision.
Schabowski announced at his 7pm press conference that East Germans would be permitted to visit the West, and a reporter (reportedly Tom Brokaw) asked when that would take effect. Schabowski paused, because the note had no details about that.
He then answered: "As far as I know — effective immediately, without delay."
The press conference was shown on East German TV, and the news quickly spread. Thousands of East Berliners went to the Wall's border crossings and demanded that the border guards let them through. With no instructions and no one willing to use force, at 11:30pm, the secret police commander ordered the checkpoints opened.
People poured through and began chipping off souvenirs of the wall. That weekend, the regime bulldozed new border crossings through the wall, with complete dismantling beginning the following summer as East Germany began to cease to exist.
Many of Schabowski's colleagues suspected he was a Western agent, but it really seems to have just been a mistake on his part. He was sentenced to 3 years in prison for his role in state murder of people fleeing East Germany, though he was pardoned after a year.