These come from the
g_return_if_fail() checks at the beginning of many
GTK+ functions. (They will only appear if your copy of
GTK+ was compiled with debugging turned on---and
hopefully it was if you are writing an application.)
You will need to look at the exact assertion that
failed to see what causes the warning. A common one: if
you accidentally access a destroyed widget or object,
you will have a pointer to memory garbage. Among other
things, this means the type tag will be invalid; so
GTK+'s runtime type checks will fail.
