"programming error: Going back in MIDI time"

[Ed Ravin]

I get these messages at the end of every compilation.  Platform is
Mac OS 10.3.9.  Any idea what this means?

GNU LilyPond 2.10.29
Processing `southern.ly'
Parsing...
Interpreting music... [8][16][22]
Preprocessing graphical objects...
Interpreting music... 
MIDI output to `southern.midi'...
programming error: Going back in MIDI time.
continuing, cross fingers
programming error: Going back in MIDI time.
continuing, cross fingers
programming error: Going back in MIDI time.
continuing, cross fingers
programming error: Going back in MIDI time.
continuing, cross fingers
programming error: Going back in MIDI time.
continuing, cross fingers
programming error: Going back in MIDI time.
continuing, cross fingers
Layout output to `southern.ps'...
Converting to `southern.pdf'...

The score block that generates the MIDI output is below.

\score { % no layout, just MIDI, unfold repeats
        {
        <<
      \new Staff = "singer"
                <<
                \new Voice = "vocal" {
                                \autoBeamOff
                                \melody
                        }
        >>
      \new Lyrics \lyricsto vocal \new Lyrics { \text }
      \new PianoStaff = "piano" <<
                #(set-accidental-style 'piano)
        \new Staff = "upper" { \unfoldRepeats \upper }
        \new Staff = "lower" { \unfoldRepeats \lower }
      >>
    >>
        }

        \midi {
                \context {
                \Score
                        tempoWholesPerMinute = #(ly:make-moment 72 4)
                }
        }