I was getting this weird error whenever switching from a given state to it's parent state in a Flex 3.5 based project.

RangeError: Error #2006: The supplied index is out of bounds.
	at flash.display::DisplayObjectContainer/addChildAt()
	at mx.core::Container/addChildAt()
	at mx.effects::EffectManager$/removedEffectHandler()
	at Function/http://adobe.com/AS3/2006/builtin::apply()
	at mx.core::UIComponent/callLaterDispatcher2()
	at mx.core::UIComponent/callLaterDispatcher()

It threw me for a minute because I hadn't made any changes to effects since I last tested the application and I couldn't see any connection between the code I had just written and any effects in the app. But after hunting around I found the culprit. There's a set of components in a Canvas that gets removed when moving to the parent state. What I had done was separate those components into two Canvases(Canvi?). For whatever crazy reason the new second Canvas can't have a RemoveEffect. The code works fine if just the first Canvas has it, but if both or just the second Canvas has it then it throws that error.

side note: the reason it took me a while to find the source of the error was because I copy/pasted the canvas declaration only changing the id, and I forgot that there was a removedEffect associated with it.

But wait there's more. The reason I split the components into two distinct Canvases was so I could position one below and the other on top of a third major component in z-space. The solution was to add the first Canvas as a "firstChild" and the second Canvas as a "lastChild". That it seems was the problem. In mxml when changing states you apparently can't add a firstChild before adding a lastChild. so I copy and pasted the first Canvas below the second one, so that all the lastChild additions occurred before all the firstChild additions and voila, presto, it works.

The reason is that when you move from a state back to its parent state it follows the order in which you add components in the state declaration to remove them. If the first component you add is added as a firstChild then that get's removed first changing the indexes and number of children of the parent container. I guess the underlying state changing function already calculated what the lastChild index was, so when trying to remove a Child with the pre-calculated index of lastChild it triggered an index out of bounds error.