After making my first Mono bug report ever, I made a patch to fix the bug :). The problem was the incorrect handling of the number of points when drawing a curve with specified number of line segment. Here it is:

Index: mcs/class/System.Drawing/System.Drawing/Graphics.cs

— mcs/class/System.Drawing/System.Drawing/Graphics.cs (revision 53896)
+++ mcs/class/System.Drawing/System.Drawing/Graphics.cs (working copy)
@@ -387,8 +387,10 @@
throw new ArgumentNullException (“points”);

Status status;

• int numberOfPoints = numberOfSegments + 1;
• status = GDIPlus.GdipDrawCurve3 (nativeObject, pen.nativeObject,
  – points, points.Length, offset,
• points, numberOfPoints, offset,
  numberOfSegments, 0.5f);
  GDIPlus.CheckStatus (status);
  }
  @@ -401,8 +403,10 @@
  throw new ArgumentNullException (“points”);

Status status;

• int numberOfPoints = numberOfSegments + 1;
• status = GDIPlus.GdipDrawCurve3I (nativeObject, pen.nativeObject,
  – points, points.Length, offset,
• points, numberOfPoints, offset,
  numberOfSegments, tension);
  GDIPlus.CheckStatus (status);
  }
  @@ -416,8 +420,10 @@
  throw new ArgumentNullException (“points”);

Status status;

• int numberOfPoints = numberOfSegments + 1;
• status = GDIPlus.GdipDrawCurve3 (nativeObject, pen.nativeObject,
  – points, points.Length, offset,
• points, numberOfPoints, offset,
  numberOfSegments, tension);
  GDIPlus.CheckStatus (status);
  }