Sweep bug maybe?

 From:  Michael Gibson
414.2 In reply to 414.1 
Hi Kreten - that's the result of using the "flat" option with a curve that has parts of the curve tangent pointing up close to the world z axis.

Basically, the flat style is not intended to be used with curves that are oriented in that direction.

What flat does is it will only swing the profile around the world z axis, but it will try to point it as much as possible in the direction of the curve. But when the direction of the curve approaches the world z axis, this causes exaggerated swings.

Here is a post with some more information: http://moi3d.com/forum/index.php?webtag=MOI&msg=273.2

- Michael