Martin Owens, 

1. activate the rectangle tool
2. move the mouse over the canvas and hold down the left mousebutton
3. (do not release the mousebutton),
   move the mouse so that a rectangle is drawn
4. press ctrl + z
5. in r12945, inkscape crashes and the rectangle cursor gets stuck throughout the OS (linux mint 15 32bit), 
the terminal outputs: 
** (inkscape:4255): WARNING **: Incomplete undo transaction:

** (inkscape:4255): WARNING **: Event: Set attribute style to "fill:#808080;fill-opacity:1" on #<Element:0x0xaf50770>

** (inkscape:4255): WARNING **: Event: Added #<Element:0x0x91bcaf0> to #<Element:0x0xaf50770> after beginning

** (inkscape:4255): WARNING **: Event: Set attribute id to "rect3262" on #<Element:0x0xaf50770>

** (inkscape:4255): WARNING **: Event: Set attribute width to "0" on #<Element:0x0xaf50770>

** (inkscape:4255): WARNING **: Event: Set attribute height to "0" on #<Element:0x0xaf50770>

** (inkscape:4255): WARNING **: Event: Set attribute x to "0" on #<Element:0x0xaf50770>

** (inkscape:4255): WARNING **: Event: Set attribute y to "0" on #<Element:0x0xaf50770>

** (inkscape:4255): WARNING **: Event: Set attribute transform to "translate(-7.3563764e-6,0)" on #<Element:0x0xaf50770>

** (inkscape:4255): CRITICAL **: void SPObject::requestDisplayUpdate(unsigned int): assertion `this->document != NULL' failed

Program received signal SIGSEGV, Segmentation fault.
0x081e6154 in ?? ()



And in v0.48.4 r9939, inkscape does not crash,
but the terminal outputs:
** (inkscape:4316): WARNING **: Incomplete undo transaction:

** (inkscape:4316): WARNING **: Event: Set attribute style to "fill:#808080;fill-opacity:1" on #<Element:0x0xac779f0>

** (inkscape:4316): WARNING **: Event: Added #<Element:0x0x8f855f0> to #<Element:0x0xac779f0> after beginning

** (inkscape:4316): WARNING **: Event: Set attribute id to "rect2989" on #<Element:0x0xac779f0>

** (inkscape:4316): WARNING **: Event: Set attribute width to "0" on #<Element:0x0xac779f0>

** (inkscape:4316): WARNING **: Event: Set attribute height to "0" on #<Element:0x0xac779f0>

** (inkscape:4316): WARNING **: Event: Set attribute x to "0" on #<Element:0x0xac779f0>

** (inkscape:4316): WARNING **: Event: Set attribute y to "0" on #<Element:0x0xac779f0>

(inkscape:4316): GLib-GObject-WARNING **: invalid uninstantiatable type `(null)' in cast to `SPRect'

** (inkscape:4316): CRITICAL **: void sp_rect_position_set(SPRect*, gdouble, gdouble, gdouble, gdouble): assertion `SP_IS_RECT(rect)' failed

(inkscape:4316): GLib-GObject-WARNING **: invalid uninstantiatable type `(null)' in cast to `SPRect'

** (inkscape:4316): CRITICAL **: void sp_rect_position_set(SPRect*, gdouble, gdouble, gdouble, gdouble): assertion `SP_IS_RECT(rect)' failed

(inkscape:4316): GLib-GObject-WARNING **: invalid uninstantiatable type `(null)' in cast to `SPRect'

** (inkscape:4316): CRITICAL **: void sp_rect_position_set(SPRect*, gdouble, gdouble, gdouble, gdouble): assertion `SP_IS_RECT(rect)' failed

(inkscape:4316): GLib-GObject-WARNING **: invalid uninstantiatable type `(null)' in cast to `SPRect'

** (inkscape:4316): CRITICAL **: void sp_rect_position_set(SPRect*, gdouble, gdouble, gdouble, gdouble): assertion `SP_IS_RECT(rect)' failed

(inkscape:4316): GLib-GObject-WARNING **: invalid uninstantiatable type `(null)' in cast to `SPRect'

** (inkscape:4316): CRITICAL **: void sp_rect_position_set(SPRect*, gdouble, gdouble, gdouble, gdouble): assertion `SP_IS_RECT(rect)' failed

(inkscape:4316): GLib-GObject-WARNING **: invalid uninstantiatable type `(null)' in cast to `SPRect'

** (inkscape:4316): CRITICAL **: void sp_rect_position_set(SPRect*, gdouble, gdouble, gdouble, gdouble): assertion `SP_IS_RECT(rect)' failed

(inkscape:4316): GLib-GObject-WARNING **: invalid uninstantiatable type `(null)' in cast to `SPRect'

** (inkscape:4316): CRITICAL **: void sp_rect_position_set(SPRect*, gdouble, gdouble, gdouble, gdouble): assertion `SP_IS_RECT(rect)' failed

(inkscape:4316): GLib-GObject-WARNING **: invalid uninstantiatable type `(null)' in cast to `SPRect'

** (inkscape:4316): CRITICAL **: void sp_rect_position_set(SPRect*, gdouble, gdouble, gdouble, gdouble): assertion `SP_IS_RECT(rect)' failed

(inkscape:4316): GLib-GObject-WARNING **: invalid uninstantiatable type `(null)' in cast to `SPRect'

** (inkscape:4316): CRITICAL **: void sp_rect_position_set(SPRect*, gdouble, gdouble, gdouble, gdouble): assertion `SP_IS_RECT(rect)' failed

(inkscape:4316): GLib-GObject-WARNING **: invalid uninstantiatable type `(null)' in cast to `SPRect'

** (inkscape:4316): CRITICAL **: void sp_rect_position_set(SPRect*, gdouble, gdouble, gdouble, gdouble): assertion `SP_IS_RECT(rect)' failed

(inkscape:4316): GLib-GObject-WARNING **: invalid uninstantiatable type `(null)' in cast to `SPRect'

** (inkscape:4316): CRITICAL **: void sp_rect_position_set(SPRect*, gdouble, gdouble, gdouble, gdouble): assertion `SP_IS_RECT(rect)' failed

(inkscape:4316): GLib-GObject-WARNING **: invalid uninstantiatable type `(null)' in cast to `SPRect'

(inkscape:4316): GLib-GObject-WARNING **: invalid uninstantiatable type `(null)' in cast to `SPObject'

** (inkscape:4316): CRITICAL **: SPObject* sp_object_ref(SPObject*, SPObject*): assertion `SP_IS_OBJECT(object)' failed

** (inkscape:4316): CRITICAL **: SPObject* sp_object_unref(SPObject*, SPObject*): assertion `SP_IS_OBJECT(object)' failed


That happens everytime i try it in both versions.