Oh, you're going to love this! It's not a bug in web2py (as I suspected). But it's also not a bug in my code. It's a bug in Windows 7!
The problem is that Windows Explorer (the file browser) is not refreshing consistently, depending on which folder view you're using. I was using the tiles view to see the contents of a folder without having to go into the folder, so I could see whether there was an uploaded image in the target folder. When the image got deleted, the folder still showed a thumbnail of the original uploaded file, even after issuing a refresh command. Conclusion -- web2py is higher quality than Windows? LOL!
