I thought I saw a ... bug (in partcombine)

[arie-lily]

I encountered some following strange behaviour from \partcombine, with this
snippet all works well:

\version "1.6.10"
\score {
    \context Voice = voiceOne {
        \partcombine Voice
        \context Thread = threadOne \notes \relative c { c8 d e }
        \context Thread = threadTwo \notes \relative c { e8 d g }
    }
}

That is to say: ly2dvi complains about (and consecutively ignores them)
clashing notecolumns, but displays the music correctly.
However, when I change 
\relative c
into
\relative c'
or
\relative c''
(of course both of them have to be changed at the same time with the
same amount), then the stems of the third notes are both printed.
I think this must be a bug, not?
Anyway, it refuses to typeset correctly a music piece that I was willing
to make with pairs of voices in altos and tenors :(

arie