This is perhaps the most bizarre problem I've ever come across, but my home directory in Ubuntu 9.10 seems to be occasionally reverting to a previous state.
For example, I'll change a setting in Opera (whose settings folder is in my home directory), then upon subsequent restart, may or may not have reset to what it was previously. This happens with all programs with settings in the home directory.
I have the Kubuntu desktop downloaded as well, so I am using KDE4. Any ideas?