You probably came here from my post on Stack Overflow, but if not, here is the thread:
http://stackoverflow.com/questions/24834797/simple-general-raycasting-in-box2d-javascript-example

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Problem 1: Raycast callback itself was being called instead its ReportFixture function.

In b2World.prototype.RayCast

Before:
	return callback(fixture, point, output.normal, fraction);

After:
	return callback.ReportFixture(fixture, point, output.normal, fraction);

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Problem 2: fraction returned by raycast callback was always being used, when it should
only be used if it is positive. 

In b2DynamicTree.prototype.RayCast

Before:
            subInput.maxFraction = input.maxFraction;
            maxFraction = callback(subInput, node);
            if (maxFraction == 0.0) return;
            if (maxFraction > 0.0) {

After:
            subInput.maxFraction = maxFraction;
            var value = callback(subInput, node);
            if (value == 0.0) return;
            if (value > 0.0) {
               maxFraction = value;

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Problem 3: For 'edges' made with polygons and SetAsEdge, sometimes floating point 
precision caused the ray to miss the edge.

In b2PolygonShape.prototype.RayCast

Before:
	if (upper < lower - Number.MIN_VALUE) {

After:
	if (upper < lower - 1.19209290e-7) {