Oh, I see.
You're setting
flyTreeView.NodeCheckedChanged += ....
at Page_Load (fired from OnLoad).
But all node-level events are fired at LoadPostData stage, one by one, all client-side actions are applied to FlyTreeView.
So you need to move your event handler assignment to ASPX markup or to OnInit:
protected override void OnInit(EventArgs e)
{
base.OnInit(e);
flyTreeView.NodeCheckedChanged += new FlyTreeNodePropertyChangedEventHandler(flyTreeView_NodeCheckedChanged);
}
void flyTreeView_NodeCheckedChanged(object sender, FlyTreeNodePropertyChangedEventArgs e)
{
throw new Exception(\"The method or operation is not implemented.\");
}
Regular events (SelectedNodeChanged, etc.) are fired after OnLoad, but all NodeSOMEPROPERTYChanged - are events of a special type, fired one by one at LoadPostData.