If you get an error like this in an actionscript 3 Flash file:

TypeError: Error #1034: Type Coercion failed: cannot convert flash.display::SimpleButton@61422481 to [symbolname].
at flash.display::Sprite/constructChildren()
at flash.display::Sprite()
at flash.display::MovieClip()
at classname()[/path_to/as/classname.as:34]

Check the symbol named in [symbolname] to make sure it is specified as a Movie Clip rather than a button.

Other possible reasons for the error are given in:

as 3.0 error #1034