There is a very common situation when user can get his file corrupted by simply copy-pasting layers from one file to another. This happens if layer contains exported values (note that bones are exported values too!).

Steps to reproduce this situation:

1. Launch Synfig and create new file.
2. Create a simple Circle Layer.
3. Select Circle Layer, right-click its Radius parameter and select "Export".
4. Enter name for exported value, i.e. "test". Click Export.
5. Save file as "file1.sifz"
6. Hit Ctl+C to copy circle layer to clipboard.
7. Create a new file in other tab.
8. Hit Ctrl+V to paste circle from clipboard. Circle will appear on workspace. Notice, that layer is copied, but it is still references exported value from "file1.sifz"
9. Save file as "file2.sifz"
10. Quit Synfig.
11. Now remove (or rename) "file1.sifz".
12. Launch Synfig and try to open "file2.sifz".
13. Error:
    

@rodolforg @ice0 @FirasH I think this is a top-priority issue to solve for next stable release (1.4.1 or 1.6.0).

What should be done here:

1. Change how Synfig deal with pasting layers when layer has exported values.
2. Change how Synfig handles situations with broken links to exported values in external documents.