Note: This is fixed in 9.0.1. In Mathematica 9, if I display any large SparseArray object in the default way (which looks something like ...
In a nutshell, why does this work (change y and see how the value updates) ...
A very abridged example of what was originally a major leak ...
With the following Dynamic cell, I can reproducibly crash Mathematica, so do not try this unless you have saved all your work! ...