Flash has been released several times, each time adding to the body of tools and objects Flash creators can use to make their movies. With each new version, certain features are created that make using an older version of the Flash player less-than-optimal.
The IMAGE Lab has Flash 5, which includes many features that don't work on Flash 4 players. When you're coding, you should consider the ramifications of using different coding techniques, and the viability of such techniques with older versions of the Flash player. (Consult the Flash ActionScript Manual if you're curious about compatibility issues -- each feature includes documentation about which versions of Flash support it.)
Of the two movies below, the one on the left is a single frame, while the one on the right is two frames long. Below the movies are the codes included in the button that makes the object disappear or reappear.
Flash 5 compatible
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Flash 4 compatible
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If you're on a computer using Flash 5 or higher, both movies will work properly. On the other hand, users with Flash 4 or lower will only be able to make the right-hand image disappear.