Can’t save your Word document? Used equations recently? Thought so.
Don’t panic though – the problem is simply with soft-carriage-returns (i.e. using Alt+Enter) in the equations.
To fix things up, remove all soft carriage-returns from your equations – making sure to include those still left behind in the tracked changes! (just accept the changes involving equations, or accept all changes).
Further details can be found here: http://answers.microsoft.com/en-us/office/forum/office_2007-word/equation-editor-causes-crashes-in-word-20102007/ – look for the post from bryan p.