Off the cuff, I'd point to any format for which you'd have to fire up an emulator to run the software to read it. I would also include any format which is not openly documented or which requires a propriety tool to view it. When you look at archiving digital formats, you have to look at the whole toolchain and realize that the substrate of the medium itself will degrade, the electrical or magnetic field will degrade, the storage format will quickly become obsolete, the digital file format will be and so forth. More relevant to the discussion at hand, supposing you do intend to store important documents in a digital format as a long-term archive. What format do you store the instructions for reading and decoding the archive? Wouldn't carbon pigment on vellum be stable enough?

Edit: On further consideration, I think we have to regard digital media and formats like a form of encryption (a simile, I don't mean this literally) which gets stronger over time. Time will bury extant evidence of what is contained on the media and how it is stored. Why should not make the information as explicit (plain-text) as possible? People in 800 years will still have to spend their efforts decoding early 21st century English legalese.