Quite simply, the option in Finder prefs to show all filename extensions is checked, but sometimes when I save something, e.g. a text file, and don't explicitly specify a filename in the save dialog, the file appears in the finder without an extension.
Yet Finder recognizes its type as SimpleText in list view, its icon becomes a preview of the document with the footer "TXT," and Quicklook displays the file correctly.
I have also tried unchecking and rechecking that box after creating the file; the extension does not appear after recheck or even relaunch of Finder. If there's nothing that can be done, that's a satisfactory answer--typing .txt is not a big deal – but I'm just curious! OS X 10.6.7.