Does a prosecutor's improper argument at trial need to be re-examined?

This court's decision in Hein v. Sullivan, 601 F.3d 897 (9th Cir. 2010), lists the following factors that we may consider in determining whether a prosecutor's improper argument requires a new trial:

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Id. at 912-13. Furthermore, an improper argument made during rebuttal is all the more effective because the defense is given no chance to refute it. See United States v. Sanchez, 659 F.3d 1252, 1261 (9th Cir. 2011) ("[T]he prosecutor presented the [improper] 'send a memo' rhetoric during his rebuttal, thus ensuring that it was the last argument the jury heard before going to the jury room to deliberate. This timing increased the risk that the inflammatory statement would improperly influence the jurors.").