We all now that Flex is very much like Java. Do not forget then that in Java the expressions are evaluated from left to right. So, if you have an expression:

a = a && b;


and a is false, b will not be looked at. This is important to remember in case when b is some expression and you want it to be evaluated no matter what a is:

a = a && anImportantUpdateFunction();


If a is false, then anImportantUpdateFunction() will not be called.

if using cfif tags, remember about the scope of defined variables (cfquery and such)

sometimes, out of the blue, you get this error message:

[RPC Fault faultString="HTTP request error" faultCode="Server.Error.Request" faultDetail="Error: [IOErrorEvent type="ioError" bubbles=false cancelable=false eventPhase=2 text="Error #2032: Stream Error. URL: http://localhost/CFIDE/test_user/returncfml.cfm"]. URL: http://localhost/CFIDE/test_user/returncfml.cfm"]
at mx.rpc::AbstractInvoker/http://www.adobe.com/2006/flex/mx/internal::faultHandler()
at mx.rpc::Responder/fault()
at mx.rpc::AsyncRequest/fault()
at ::DirectHTTPMessageResponder/errorHandler()
at flash.events::EventDispatcher/flash.events:EventDispatcher::dispatchEventFunction()
at flash.events::EventDispatcher/dispatchEvent()
at flash.net::URLLoader/flash.net:URLLoader::redirectEvent()

Flex documentation< (need link) offers no explanation, one blog gives a short description. So what is it then?
- check if you have mistyped the URL (a page, your *.cfm file, etc)
- if you are pointing to a *.cfm, check that you are using the right Data Source to access the DB
- check if the Data Source connects to the DB you need
- check that your SQL query does what it is supposed to, i.e. does not insert rows w/o specifying primary key, or adding columns that do not exist in a table, etc.