That is:
If a movie is loaded with the URL:
movie.swf?a=initialValue

and the movie contains a script that does something like this:
_level0.a = "some other value"

the value of a will get reset to “initialValue” on every frame, until the movie finishes loading!

This is different than if you pass in the variable using flashVars. If you use flashVars, the variable stays set, as you would expect it to.

Here are some tests to demonstrate the behaviour. It’s best to clear your cache in between each test, or you will get varying results.

• Using query string variables
• Using flashVars

Here’s a screenshot from a test using query string variables:

The test movie is embedded with code that passes the variable “a” set to “from html“. The movie continually displays the value of the variable “a” in the scrolling text field. On the fourth frame, it changes the the value of a to “set from script.”

At any time, you can click on the “Set” button to set the variable to “set from script.” When the movie is finished loading (there’s a 700K JPEG file on the last frame to make the movie large and slow-loading), you can click the “Stop” button to stop the scrolling text field from updating.

Play with both tests, and compare the effects if you clear the cache between tests or not, and try clicking “Set” at different times.

UPDATE: This only seems to occur in movies published for Flash Player 8! Here’s a test using a movie published for Player 7 and variables passed via the query string:

• Using query string variables, Flash Player 7

And, of course, I have no idea how ActionScript 3/Flash Player 9 behaves in a similar situation…