I have some flat triangles in my FEM mesh (angles for example 30, 30, 120). I wonder whether this just decreases the mesh quality, or whether it will really cause problems.Why do I ask? When computing the matrix for the Laplacian for linear elements, the off-diagonal elements are positive, and the diagonal elements are negative. In the case of flat triangles, this might change. Seems to be a bit suspicious to me...
FEM with flat triangles
finite element