The Thinner Disaster is a plot point in Epic Mickey. It was accidentally caused by a mischievous Mickey Mouse before he started to gain lots of worldwide popularity. It was the dawn of Shadow Blot and his Blotlings. It brought despair and ruins over all of the Cartoon Wasteland.
A virtually unknown Mickey Mouse, out of curiosity, enters Sorcerer Yen Sid's workshop through a mirror in his house and discovers the model of the land Yen Sid created and the tool used to create it, the magic paintbrush. Fiddling with the brush and some paint to make a self-portrait, Mickey accidentally creates the Shadow Blot. Panicking, Mickey quickly tries to erase the Blot by throwing paint thinner onto it, but spills more paint on the model in the process. Upon seeing Yen Sid approaching, Mickey quickly tries to clean up the mess, but in his haste, spills thinner onto the paint spillage as he flees back to his house, while the Blot, having survived Mickey's attempt to destroy it, enters through a portal created by the paint/thinner mixture (taking the Jug - the bottle of thinner - with him) and takes control of the ruined world from its first resident, Oswald ("The world I had created was ravaged - a wasteland").
In the game there are four times the Thinner Disaster is mentioned. The first time is when Mickey enters Mean Street, the second time the Thinner Disaster is mentioned is in Mickeyjunk Mountain, when Mickey discovers that Oswald runs the Wasteland, the third time is when Mickey meets Oswald, this time the disaster is not technically mentioned, Oswald drops a bottle of Thinner to the floor and suddenly Mickey remembers that he caused the Thinner Disaster. The fourth and last time the disaster is mentioned by Mickey when Oswald offers a restart in their friendship relation, and when Mickey was going to shake his hand with Oswald's, he confesses he caused the Thinner Disaster.