This commit introduced the issue:

commit f89caeff5c1820caecfd2e1d9b93ffeb0a477a17
Author: Arun Raghavan <arun.raghavan@collabora.co.uk>
Date:   Tue May 25 01:04:43 2010 +0530

    qtdemux: guess bitrate if only one stream's bitrate is unknown
    
    If the bitrates for all but one audio/video streams are known, and the
    total stream size and duration can be determined, this calculates the
    unkown bitrate as (stream size / duration) - (sum of known bitrates).
    While this is not guaranteed to be very accurate, it should be good
    enough for most purposes.
    
    For example, this is useful for H.263 + AAC streams where no 'btrt' atom
    is available for the video portion.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=619548

Assertion is:

g_assert (size > qtdemux->header_size);

In the case of the Apple Trailers I tried (with WebKitGtk), size and qtdemux->header_size are equal.
The issue can likely be reproduced with Totem, for instance playing http://trailers.apple.com/movies/independent/larafle/larafle-tlr1_h480p.mov