After upgrading to Ubuntu 11.10, I once right-clicked on a folder and told it to open the folder with Movie Player ("Open with other application...").
No harm done there, right? Nope, now Ubuntu opens folders with Movie Player instead of Nautilus. If I plug in a thumb drive, Movie Player opens.
If I click on the trash icon... Movie Player! The default action for opening a folder has somehow become Movie Player. Clicking on the "Home Folder" icon still opens Nautilus and opening a folder in Nautilus stays in Nautilus, but other links to folders automatically open Movie Player. How can I change this back?