Guess 4 (maybe not so wild): If you pasted to Storage Card, then there's the possibility that you had an interruption in data flow. This can happen when a physical connection is compromised between any of the little fingers that touch on the card's gold terminal pads. I had such a problem awhile ago, and it got worse. But it stopped after I cleaned the card terminals with a soft dry lintfree cloth. Since then, I've been extra careful about inserting card, making sure to hear and feel a clean click. And removing the tiny little card as smoothly as possible (while struggling to hold the damnable side door open).
Here's screenshot of what I saw before cleaning terminals: