Beginner's mistakes

Has anybody considered a FAQ of mistakes made by beginners? Or are
people's mistakes too idiosyncratic to bother?

Here's my contribution:

1. The default scope for a class you create is "internal": it can be
seen by other classes in the package, but not outside the package. To
make it visible throughout your project, declare it:
public class className {...}

Figuring this one out was especially hard because the following doesn't
generate an error message:

import packageName.className;
(where className is actually internal to its package)

Instead, the compiler flags every use of className inside your code,
leaving you (or at least me) scratching your head... "But I imported it..."

2. Functions need a return type, even if they don't return a value:
public function name(...): void

3. Don't forget to use "var" when declaring the local variables in a
function. The error message from this one is a little mystifying.

4. Accessing mxml components: In examples in published books, mxml
components are used as if they were global variables. For example,
<s:Group id="mainmenutop" visible="false" left="0" top="0"
width="100%" height="100%">
...
</s:Group>
The example simply refers to "mainmenutop" as if it were a global
variable. It isn't. It's an instance variable in the Application
object. So if you want to refer to these in other classes, you either
need to provide access functions in the Application object, or pass them
to the object that will use them.

In the above example, I have a "GameSequence" class that will show or
hide the various components that make up the top-level mxml file, as the
user goes from one part of the game to another. So I will either need
to provide accessor functions:
public function getMainmenutop():Group { return mainmenutop; }
or pass them to the sequencer:
sequencer:GameSequence = new GameSequence();
sequencer.addPage("mainmenu", mainmenutop);

Btw, I'm soliciting comments on which of the above is less ugly from an
OO point of view, and or other suggestions for structuring this code.