Every once in awhile when I try to save a document/image/whatever... in Adobe Photoshop, I get an error in the form of a pop-up that says, "Could not complete this operation because an assertion has failed". The first time I tried to figure out how to fix this, I ended up in forums suggesting all kinds of things from re-booting; to making changes in the registry to, of course, blaming viruses and getting the right protection.
The simple solution for me... has been (drum roll):
Hold down Ctrl + Alt when clicking on "save for web" (in file menu) it'll ask you if you want to reset preferences - say yes.
Should fix the problem.