Strange errors in cgierror.log

I just noticed I have a bunch of these in my var/log/hobbit/cgierror.log :

$ head cgierror.log
*** attempt to put segment in horiz list twice
*** attempt to put segment in horiz list twice
*** attempt to put segment in horiz list twice
*** attempt to put segment in horiz list twice
*** attempt to put segment in horiz list twice

Any idea what this is caused by? I searched for that string in the 
hobbit source code and found nothing.

-Charles