Hi,
under some circumstances you can damage your symbols / your scene.
Watch the attached video to see how to reproduce the issue.
But the steps should be something like the following:
1. create a rect
2. make it a symbol (get rid of it from your artboard)
3. use the symbol e.g. 3 times by dragging out of the lib
4. group the 3 symbols
5. make the group a symbol as well
6. drag that group-symbol once again by dragging it from the lib, so you have 2 group-symbols
7. drag the single rect-symbol out of the lib
8. modify the hierarchy by dragging the single rect-symbol into one of the group-symbols to make it a child object
9. delete that new child object (the rect-symbol)
Cheers, Stefan.
@MEB: Maybe this was the reason how I damaged my file (reported bug "AD crashes when moving artboard around" ?!?
AD_SymbolFckUp.mov