Some actions have visible results (moving or resizing a component) in design
mode and others (adding event listeners and some properties) are not immediately
visible.  When I type CTRL-Z mutiple times, I often end up undoing things I
don't intend to undo.  (This is exacerbated by bugs in UNDO, but would still be
a problem.) 

I'm not sure what the best way to visualize these hidden operations.  Tomas
suggested showing a message in the status bar about the last undone/redone
operation.  Another possibility would be to do something like Word, where it
lists the operation in the menu next to Undo/Redo: i.e. "Undo Typing," "Undo Clear"