Create a shape such as a rectangle
Convert it to curves
Turn it into a symbol
Select the shape inside the symbol in the layers panel
Select the node tool, with [A]
Click on an edge to insert a point in the shape. It will fail to do so, selecting one of the existing points instead.
Undo.
Depending on which edge you inserted the point into, it will either remove one of the original points, and create a desync from the symbol (the dashed orange line in the layers) even with sync enabled, or crash the program (probably by trying to remove an out of bounds point index, because it thought a point was added but it wasn't).