The stories of what happened to the 60s Zappa/mothers tapes,and the Bob Marley 1977 "bedroom" tapes are legendary.The tapes literally flake apart and fall to pieces.

Interesting article,  it specifically talks about remastering "Frampton Comes Alive",but there's a lot more.Here's an excerpt:

Surround Audio Might Require Some Detective Work—And Having a Good Lawyer's Not a Bad Idea, Either
By Dan Daley

As the major labels gear up to feed the
consumer pipeline with DVD-A product, most will discover that there are
often major hidden pitfalls when it comes to reformatting archival
material. After sitting in a vault for two decades, many so-called
master tapes are in poor physical condition, and if the tapes have been
improperly stored or cataloged, then anything from a guitar solo to an
entire reel of tape can go missing.

"The problem in
the music business is that 25 years ago, no one had any concept about
the future uses of the recordings," observes Blaine Graboyes, founder
and creative director of Zuma Digital. The multimedia authoring and
audio facility in New York City has been doing 5.1 remixes for a range
of media, including film, television, advertising and music videos.
"Now you pull a tape out—if you can find it—and you find five takes of
the same song. They all sound the same to you; only one is the approved
version, and the engineer's been dead for 20 years," Graboyes explains.

Graboyes applies terms picked up from Zuma's
corporate clients to describe the problems that the music industry
faces as DVD-Audio attempts the transition from a novelty to a
mass-market product. "The biggest issue in doing any kind of
repurposing of previously used creative elements is `knowledge
management'—the ability to know where all the assets of a project are
and how to retrieve them," he says. "And the absolute biggest problem
that surround music faces on a day-to-day basis is missing assets, from
pieces of recordings to the media itself. The second biggest problem is
missing records of the asset. That is, the documentation that tells you
things like how to use the asset, which pieces the artists intended to
be used and which were not, and where to find the right pieces on the
media. Without either of those things, you're not going to be
completely successful in repurposing them for new media."

The stories of tapes moldering away forgotten
in broom closets, dumped in the trash or auctioned off for a few
dollars when studios and record labels close down are legion in the
entertainment business. Proper archiving and record-keeping of the
music media has been almost nonexistent. And that's compounded,
Graboyes adds, by the fact that the entertainment industry, in general,
is a transient one. "The turnover rate in the music and film businesses
is incredibly high," he says. "One of our clients has had four
different assistants, all of whom get up to speed on where things are
and then leave, and the next person has to start all over again." 


It's not just the golden oldies that are
missing the most in music. Even relatively recent recordings show up
with missing pieces, slowing down or completely halting the remixing
process. Jake Nicely, co-owner of Seventeen Grand Recording in
Nashville and one of the leading multichannel remixers in the music
industry, was contracted this year to do a DVD-Video multichannel remix
of a live album by the country group Alabama. Though the record was
originally released in stereo in 1998 on RCA Records, pieces of the
master recording quickly went missing, and Nicely had to turn to
gumshoe detective techniques to get the project moving.

--- On Wed, 8/27/08, Charles Lawson <[log in to unmask]> wrote:
From: Charles Lawson <[log in to unmask]>
Subject: Re: [ARSCLIST] ELP Turntable  (Re(2): [ARSCLIST] RIAA EQ software)
To: [log in to unmask]
Date: Wednesday, August 27, 2008, 12:09 PM

"Michael H. Gray" <[log in to unmask]> writes:
>I'm curious to know how master tapes 'deteriorate' Is it in the
>carrier or in the magnetic signal?

Both.  Depending on tape stock, materials and storage, the tape itself may
degenerate over time causing drop-outs or worse.  (There have been many
extensive discussions of the hydrolysis problem associated with tapes from
the 70s and 80s, for example.)

Also, magnetic tape recording itself is inherently unstable.  Those
magnetic domains don't just sit there for all eternity in the same
configuration as when they were recorded.  They are easily influenced by
temperature and external magnetic fields (including that of Earth itself)
and will tend to randomize over time.  If the tape was recorded at a high
level, layers of audio will print through from one to another and this
problem tends to get worse the longer the tape is sitting there. 
High-level recordings also have a greater tendency to exhibit increasing
distortion over time.

It goes on an on...

Some references here:

and there are lots more.

People like Richard Hess have spent substantial chunks of their lives
dealing with these issues and can give more detailed explanations than I
have time for today, alas.  It's all very interesting and can take down to
quantum mechanics, if you like!  (Aside from being an expert, Richard is
also a really nice guy!  If you need a tape transferred, go to him.)

Charles Lawson <[log in to unmask]>
Professional Audio for CD, DVD, Broadcast & Internet