If you add a new frame to a sprite's animation and drag it to the front, the frame that use to be 1st will lose its image and obtain the image of the new frame when you save the project.
This bug ruins animations.
Steps to see the bug:
1. Open sprite, add a new frame.
2. Drag new frame to the front of the animation.
3. Draw something on the new frame, or load an image.
4. Save the project as new file.
5. Preview the project, look how the old frame is gone!