It is important to remember that Scan&Solve™ computes a numerical approximation of an idealized theoretical model, not physical reality. Every model has its limitations. For example, the linear theory of elasticity predicts infinite stresses near "wedges", re-entrant corners, interfaces between different materials, and so on. In physical reality, this cannot happen, because the material simply deforms more "plastically" (as opposed to "elastically"). But in the computer simulation, this means that at some points in your model, stresses will never converge – they will just get bigger and bigger as you increase the resolution. The more complex your model is, the more likely you will have some points like that.
More generally, the linear static model of elasticity (and hence Scan&Solve™) does not account for many important physical phenomena, including vibration, buckling, material and geometry non-linearities, large and plastic deformations, and so on.