The ActionScript 3.0 Documentation is Muck
Imagine, you're relatively new to Flex. You try to create an array of objects. From your Java days, you remember the java.util library contains ArrayList, which fits the bill perfectly. Now, you go to the Flex documentation, mx.collections, expecting to find something similar. The LiveDocs contain nothing, so you try Google, which give the suggestion of using mx.collections.ArrayList.
Hold on, didn't you just read the ActionScript documentation and there was no mention of ArrayList? For fun, you try importing mx.collection.ArrayList, creating an ArrayList, populating it and then print the length of the array.
var theList : ArrayList = new ArrayList();
Surprisingly, this compiles and outputs 3 as the expected result, despite the fact there is no documentation to suggest it would work.