I can undo most actions, but some (mirror modifiers, some kinds of scaling) cannot be undone. Pressing Ctrl-Z does not reverse them (the actions show up in my undo history, but nothing happens when I press Undo).
I can undo scaling done with the S key, but not scaling done using this menu:
Undo works on most other actions, just not these ones.
I know switching between Object and Edit mode can cause problems for Undo history, but I'm staying in Edit mode the whole time.
What makes these actions "special" in that they cannot be undone by normal means?