Why does my cloth simulation mesh glitch after editing its geometry?

Question

Okay this is really frustrating because it doesn't make any sense. I'm animating a cloak for my character. I run a simulation and it animates fairly well. I dissolve one line of geometry running right down the middle of it and replay it. It looks a little better.

I make a few more changes and decide to go back and add that geometry again. I just loop cut right down the middle. I play it again and it starts stretching all over the view port and glitching and clipping and tanks my fps really bad. This isn't the first time this has happened either, is this common for blender?

Another similar thing happened where I moved a couple of vertices on the collision mesh of the character wearing it. I only tweaked the position of 4 or 5 vertices from very tiny areas and the cloth modifier just forgot how to work. Again. What causes this?

Answer 1

So, I figured out what it was... I removed geometry from a vertex group that I had pinned, and added it again without reassigning the new vertices which caused everything to wig out.

Turns out the solution was as simple as adding selecting the new vertices I was adding and assigning them to the vertex group and that solved it.