Using Effects -> Bold (horiz 25, vert 20). Arial ends up with knotted contours even though the original contours are clean-cut and smooth. The knots form where a contour changes direction at an acute angle e.g. in glyphs such as 'N' and 'M'.
Using a handwriting font, which has inherently ragged contours, the problem is multiplied greatly:
Combinations of Effects and other transformations produce unpredictable and, sometimes, wierdly interesting results!
Joe.