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This often happens with shapes that are imported rather than designed inside MAX. Sometimes, the shape is not a complete shape, i.e. with a starting point coincident with the end point. One quick way to find the errors in your model is to use the Shape Check in your utilities panel. This will point you to the reason your shape is messing up.
What I generally need to do once I've imported a shape to MAX is go into Edit Spline mode, and drag a selection around each vertex on the shape. If two vertices are found (it will say so in the rollout) then weld them and continue around the shape until all coincident vertices are welded together. Do this before any extruding!
There are also cases where the normals are flipped, but the above situation is far more common.
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