Programming errors if closing beaming brace omitted

[Nick Payne]

Not sure if this indicates an error in Lilypond, but if I forget the 
terminating brace when using manual beaming, then the log contains 
several "programming error" lines and the output contains noteheads 
without stems. Here I've commented out the closing brace:

\version "2.15.43"

\relative c'' {
    a8[ \grace { g16[ a] } g8 g c%]
}

and the log contains:

===========================================
Interpreting music...
/home/nick/lilypond/examples/test.ly:4:10: warning: unterminated beam
    a8
          [ \grace { g16[ a] } g8 g c%]
Preprocessing graphical objects...
programming error: Grob direction requested while calculation in progress.
continuing, cross fingers
programming error: Grob direction requested while calculation in progress.
continuing, cross fingers
programming error: Grob direction requested while calculation in progress.
continuing, cross fingers
programming error: Grob direction requested while calculation in progress.
continuing, cross fingers
Finding the ideal number of pages...
Fitting music on 1 page...
Drawing systems...
programming error: stem::length called but will not be used for beamed stem.
continuing, cross fingers
programming error: stem::length called but will not be used for beamed stem.
continuing, cross fingers
programming error: stem::length called but will not be used for beamed stem.
continuing, cross fingers
programming error: stem::length called but will not be used for beamed stem.
continuing, cross fingers
Layout output to `test.ps'...
Converting to `./test.pdf'...
Success: compilation successfully completed
===========================================

test.png
Description: PNG image