* Lock screen in effect. This the most common cause. If scripts are very
complicated and nested, it is not unusual to have at least one too many
'lock screen' calls. If the screen is locked, visual effect commands do
nothing.
* Monitor bit depth is greater than 8 (more than 256 colors).
* Going between two stacks with different card sizes (or any two stacks
with HyperCard before 2.1).
* A card window that straddles two different displays.
This article is adapted from the Claris Tech Info database.
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