Example code is in the attachment.

When click "Delete first Item" will remove the first entry and update listbox.
SOMETIMES will occur "Error writing 'visible' on type org.zkoss.zul.Label" exception.
If remove visible="@load(foo.flag)" attribute, it will always work fine.

After preliminary analysis, (suppose first click occur error)
VM want update the listitem which index is 49, but model is already remove the first entry and size is 49,
so foo and foo.flag is null and can't transform to boolean.