Tab substitution: Some mailers will transform any tab characters you type into multiple spaces - but only at the moment you send the message (you can use the arrow keys to confirm this: see if the cursor moves more than one space when you arrow over a tab).
Detection:  Suspect this if lines are indented.
Prevention: Sender side: Turn this feature off in your mailer. Upgrade mailer/PGP to a version that runs PGP after the tab substitution.
Repair: Receiver side: Replace long runs of spaces with tabs, probably eight spaces per tab.

An error has occurred; found detached sig, no callback: Users of the Eudora+PGP plugin will occasionally see this when reading signed, nonencrypted email from users of MUTT.  (It might happen in other contexts as well, but I'm only familiar with this one.)  This is a bug on the Eudora+PGP side, although I don't know whether it's Eudora or PGP who's at fault.
Detection:  Pop-up with the text shown here.
Prevention: Get NAI or Qualcomm to fix the bug.  (I thought this was done long ago, but I just got a new case today, 1/4/2000.  Maybe it's a Y2K bug! ;-)
Repair: Receiver side: Note the name of the link in the message - the thing you just clicked to get that pop-up.  It will be similar to the message subject, though perhaps truncated or otherwise tweaked.  Let's call this Subject.  Use Eudora's "File: Open File..." command.  Select "All files (*.*)" in the drop list.  Navigate to your Eudora attachments folder, find a file named Subject.ems.  Near the top, you'll find a bit that looks something like this:
boundary=+QahgC5+KEYLbs62.
Put quotes around the value, like this:
boundary="+QahgC5+KEYLbs62"
Close and save the file.  Go back to Eudora, click the link again.
Mass repair:  A reader, John Denker, has written a Perl script to fix all the messages in your attachments folder.  I think this will only work on Windows, and you'll need Perl installed.  Perl doesn't come with stock Windows, but it's available either in the Microsoft Resource kit, or (free!) from ActiveState.

Styled-text lossage: Some mailers can give your email fancy appearance - bold charcters, underlining, different size fonts, colors, etc. Ultimately, this will be represented in some enhanced text format, like HTML. Conventionally, whenever an HTML email message is sent, there is also a plain-text copy sent. Mailers typically only give you (and hence, a manual PGP) access to one of the two forms, so the other one may still be unsigned. Further, the one the mailers give you access to is often the HTML one, but your process for clear-signing (such as via the clipboard) may only cover the text, losing the formatting. And, finally, if you paste a clear signature into a nominally HTML message, some of the characters in the signature part may get changed by the HTML processing.
Detection:  Suspect this if the lines look badly formatted—lost indentation and so on.
Prevention: Sender side: Turn this feature off in your mailer. Upgrade mailer/PGP to a version that supports PGP/MIME.
Repair: Receiver side:See the repair under Quoted-printable substitution.

Spell-checking: If you correct your spelling after doing the signature, you'll break the signature.
Detection:  Hopeless, unless you know your correspondent's favorite errors.
Prevention: Sender side: Don't do that! Or, upgrade to a mailer/PGP that runs PGP after the spell-check.
Repair: Guess which words were originally misspelled, and how.  (Yeah, right.)

Space, blank line eating: When displaying a message, some mailers delete space(s) before a line of text, or remove an empty line, or remove a line consisting only of a space.
Detection:  You might be able to find the message undamaged in the file where your mailer stores messages.
Prevention: Receiver side: It's a bug in your mailer - get your vendor to fix it!
Repair: Receiver side: Make a copy of your mail storage file, edit it with some other editor, cut it down to just the message of interest, save that to a file or copy to the clipboard, have PGP verify the result.

8-bit character loss: Any characters not contained in the basic US-ASCII character set take more than 7 bits to represent.  Some mail transports or viewers may convert these to some 7-bit character (typically, not one that makes any sense).  Some drop the character out.  Some truncate the line at that point.
Detection:  Misspelled words, lines truncated at a point where an accented letter belongs.
Prevention: Sender side: Look for a mailer setting that translates these into "quoted printables," if your recipient can handle them (but see the next section before you proceed).  Use alternate spellings. Upgrade mailer/PGP to one that runs PGP later in the process.
Repair: Receiver side: Guess what the letter or accent might have been.

Quoted-printable substitution: "Quoted printables" are a MIME trick for representing 8-bit characters and other troublesome things [A quoted printable looks like "=xx", where "xx" is a two-digit hexadecimal number for the character value.  If this is getting too deep for you, just remember "=xx".] If the sender's emailer, or any transport along the way, decides the message needs this protection, it's entitled to make these changes.  If the receiver's emailer doesn't know how to undo them, then the signature will not check.
Detection:  Receiver side: There will probably be a mail header named Content-Transfer-Encoding, that says quoted-printable.  There will be some of those "=xx" things.  (Note that normal PGP "goop," such as the signature block itself, can often contain "=xx" things; these are OK.)
Prevention: Both sides: You wouldn't be in this mess unless at least one of you has a mailer that supports MIME; if both do, then using PGP/MIME will prevent this entirely.
Sender side: Find a way to turn off quoted-printable processing in your mailer.  If you find it, it will probably come with an emphatic warning to leave it on; that's a reference to the 8-bit problem mentioned in the previous paragraph.  If you find yourself experiencing both these problems (and can't switch to MIME for some reason), the only solution is to combine the preventions: turn off quoted-printable processing (to avoid confusing your recipients' mailers), and encrypt the messages (to protect the 8-bit characters in transit).
Repair: Receiver side: This can be done, but it ain't easy.  You'll need a binary editor.  You'll need to spot all the quoted printable sequences in the message, figure out what real, 8-bit character they represent (your binary editor should help), and replace each "=xx" with the appropriate character.

Bogus quoted-printable substitution: Sometimes, things that look like quoted printables, but aren't really, get treated as if they were (translated into 8-bit characters they should never have been).  This usually arises from a botch in the configuration at the sender's end, which marks a message as having quoted printables when it actually does not (and the receiver's emailer knows what to do with them, and trustingly does it).
Note that sequences like this are common near the end of the PGP "goop" that makes up the signature, or an encrypted message!  One common way to reach this state goes like this:
1. Sender composes a message, in a MIME-competent mailer, using 8-bit characters.
2. Sender PGP-signs or encrypts the message (not using PGP/MIME), concealing the true 8-bit characters but introducing some phantom quoted printables, in the PGP goop.
3. Sender's emailer sends the message off, claiming it's been processed with quoted printables because that was needed the last time it looked (step 1).
4. Receiver's emailer "translates" some of the PGP goop into inappropriate 8-bits.
Detection:  Receiver side: Look for non-USASCII characters where they don't belong.  Check carefully near the end of the PGP goop.  If possible, save the message to a file and check it with some binary editor (see above).
Prevention: Both sides: Both your emailers appear to support MIME; see if you can use PGP/MIME.
Sender side: Find a way to turn off quoted-printable processing in your mailer.  If you find it, it will probably come with an emphatic warning to leave it on; that's a reference to the 8-bit problem or the space-eating problem  mentioned in previous paragraphs.  If you find yourself experiencing both these problems (and can't switch to MIME for some reason), the only solution is to combine the preventions: turn off quoted-printable processing (to avoid confusing your recipients' mailers), and encrypt the messages (to protect the 8-bit characters in transit).
Repair: Receiver side: This is even harder than the previous case.  You'll need one of those binary editors.  You'll need to spot all the 8-bit characters in the message, figure out their hexadecimal representation (your binary editor should help), and replace each 8-bit character with the appropriate "=xx".  Now, here's the really hard part: if the hexadecimal representation of the character includes letters, you'll have to guess whether they're upper or lower case.  The case doesn't matter to quoted-printable, but it most definitely does matter to PGP.  If there are two letters in a given quoted-printable representation, then there are four possible combinations of case.  If there are two of these two-letter quoted printables in the message, then there are sixteen possible combinations.  And so on.  Perhaps you should consider PGP/MIME again?

Jack Repenning