collision problem

[James E. Bailey]

I can't seem to reproduce this out of context, but I have a clef  
change that happens on top of a bar line. There are other clef  
changes, none of the others have collisions.
The most minimal snippet I could extract follows.
\version "2.11.57"
\include "deutsch.ly"
\paper { indent = #0 }
Vocal = \new Voice = "vocal" \relative c''{
   c4. gis8([ e']) e
   e4 a,8 f'4 h,8
   c4 r8 r4 r8
   r4 r8 r g g
}
Text = \lyricmode { nach, je -- der Knap -- pe tut mir’s nach. Und da }
RH = \relative {
   \clef bass \time 6/8
   <c e >8 <c e> r <gis e'> <gis e'> r
   <a e'>8 <a e'> r <h d> <h d> r
   \clef treble e4.( f)
   <c e>4. <h d>
}
LH = \relative a,,{
   \clef bass <a a'>8 <a a'> r <e e'> <e e'> r
   <f f'>8 <f f'> r <g! f'> <g f'> r
   <<
      \new Voice { \voiceOne
         \appoggiatura <e' g>16 \voiceOne c'4. h8\prall( a h)
         c8( g e) g4. }
      \new Voice { \voiceFour
         \shiftOff
         c,4.( d)
         c4. s }
      \new Voice { \voiceTwo
         c,2.
         c4( e8) g4. }
   >> \oneVoice
}
\score {
   <<
      \new Staff \Vocal \new Lyrics \lyricsto "vocal" \Text
      \new Staff \RH
      \new Staff \LH
   >>
}