warning not understood

[Hugh Myers]

When processing this:

\version "2.13.7"

\relative c' {
  \time 3/4
  << { \acciaccatura d'8 c4 b a } \\ { a,2. } >>
}

I get this warning message:

C:\Users\hsmyers\Desktop\Lily Projects>lilypond.exe test4.ly
GNU LilyPond 2.13.7
Processing `test4.ly'
Parsing...
Interpreting music...
Preprocessing graphical objects...
test4.ly:5:25: warning: ignoring too many clashing note columns
  << { \acciaccatura d'8
                         c4 b a } \\ { a,2. } >>
Solving 1 page-breaking chunks...[1: 1 pages]
Drawing systems...
Layout output to `test4.ps'...
Converting to `./test4.pdf'...

It engraves correctly, but clearly there is something going on here
that I don't understand. Could someone enlighten me?

--hsm