If a coded step tries to enter text into a Textbox that is contained in an IFrame, nothing happens. Sample code is this:
Actions.SetText(Pages.WholeDocument2.FrameTestingIFrame.IFrameTextBoxText, "text box in frame");

The non-coded version works as expected. As soon as you select Customize Step in Code (so you can programtically change what text will be entered), then it silently fails to enter the text.