IMPACT defines the mouse coordinates {x: 0, y: 0} so if some entity is at the point the mouseover event will be executed unexpectedly in the beginning. The coordinates should be defined with Number.NEGATIVE_INFINITY or something which is far from common numbers instead of 0 I guess.

For example 

	_overlapped: function() {
		return ig.input.mouse.x + ig.game.screen.x > this.pos.x &&
			ig.input.mouse.x + ig.game.screen.x < this.pos.x + this.size.x &&
			ig.input.mouse.y + ig.game.screen.y > this.pos.y &&
			ig.input.mouse.y + ig.game.screen.y < this.pos.y + this.size.y;
	}

This function in a sub class of Entity returns true if the entity is overlapping 0, 0 in the beginning even if the actual mouse cursor is not there.

Cheers!