UPDATE!
This bug:
1. Open project in C2 (it's a single-file project)
2. Open sprite "block"
3. Re-name animation "block7" to "sand"
4. Save project
5. Re-open the sprite and look at that animation
6. There is no visual image
Then
7. Save project
8. Open sprite "block" again
9. Re-name animation "sand" to "block7"
10. Debug without saving
11. Error!