What is it with \bar "||"?

[Patrick or Cynthia Karl]

I recently had a problem with a \bar "||" at the end of a line suppressing the 
initial repeat sign in the following repeat volta section.  The answer was to 
use some magical \bar instead which cured the problem.  It was not clear to me 
then why the \bar "||" seemed to cause the following section to be treated as a 
new piece.

I am now having a similar problem with the following code:

\version "2.18.2"

A =   \relative b' {
        f4 g g a        % 16
        \bar "||"
        (f4) f f f      % 17
}

which throws an error: syntax error, unexpected EVENT_IDENTIFIER
for the attempted slur across the double bar line.

Is there a workaround for this?  If so, should I be able to figure it out?

I still think the documentation about repeat signs only being suppressed at the 
beginning of a piece is wrong and should be fixed.  It might also be nice to 
have a better explanation why the \bar "||" has the effects it does.