OK, I can confirm there's a bug. It happens when adjacent polygon edges are almost collinear.
Here's the fix (that hasn't been heavily tested yet) at about line 4220 in clipper.cs
void OffsetPoint(JoinType jointype)
{
m_sinA = (normals[m_k].X * normals[m_j].Y - normals[m_j].X * normals[m_k].Y);
if (Math.Abs(m_sinA) < 0.00005) return; //ADD THIS LINE (todo - check this!)
else if (m_sinA > 1.0) m_sinA = 1.0;
else if (m_sinA < -1.0) m_sinA = -1.0;
Note: 0.00005 is just a value that's close enough to zero to remove the spike in your supplied sample, but it may need to be readjusted with further testing.