How to repair "assertion `height >= -1' failed"?

[Gus Koppel <gtk spamkiller bytechase cx>]

When opening some dialogs at runtime, which were created using Glade, I
get a slowly increasing number of GTK critical messages
"(gtk_widget_set_size_request): assertion `height >= -1' failed" as my
work proceeds. This apparently indicates some layout faults of widgets
inside a window / container.

However, I can't find any widgets missing or being obviously mislayouted.
There doesn't seem to be a simple way to determine  w h i c h  widgets are
complained about. I haven't found out which kind of layout in Glade
results in those faulty constructs. Maybe HPaneds / VPaneds, which I use
rather frequently? Any simple idea how to get rid of this (probably not
too serious) annoyance or how to avoid it?