The brown paper cement bag on which Eduardo draws Salvador is not the same one that Salvador discovers in the art gallery years later. The white paint that almost completely covers the paper in the childhood scene later covers only half of the large circular printed label and the "35 KG" printed at the top left is truncated.
The eye color of Penelope Cruz (Antonio's young mother) are brown while the old mother's are pale blue.
In the very last scene of the film, it appears that Penelope Cruz is an actress who plays Antonio Banderas's young mother while filming a scene in front of him.
When Eduardo draws Salvador, he sits right in front of him, but when the drawing is shown the angle is oblique a bit above from the left.