I somehow managed to clear out the name of a 2D procedural texture when closing the editing window without realizing it. (I was editing an existing texture rather than creating a new one--I think.)
Everything appeared to work fine. Object was updated with modified texture. However, when I went to further tweak the texture, its name was no longer available (for selection) in any of the lists. I saved and closed the file, re-opened it, and the file itself loaded fine. However, the object on which the texture had been applied was listed in the objects list as "unreadable" followed by the object's name.