The jury will think that the crime was a campaign finance violation (because Cohen said there was one, even though he was lying) and Smith was not allowed to inform the jury of the truth.
The nuclear option would be (if the jury finds Trump guilty) for Bragg to charge Costello with perjury for saying that Cohen was lying. Of course that would depend on whether Biden tells him to do so.
That's it in a nutshell. And that's why Merchan couldn't allow the former FEC Commissioner testify to that fact. They are going out on this extremely thin limb that Trump merely THINKING it was a crime is enough to promote the misdemeanor to a felony, and therefore Commissioner Smith's testimony was moot. Cohen litterally testified that he/Trump committed FECA crimes, and the judge prevented the defense from presenting a vigorous rebuttal.
Without Commissioner Smith testifying, the jury likely (wrongly) thinks Trump committed an FECA crime. Merchan might address this in jury instructions to try to avoid this glaring cause for appeal. I'm not sure if the jury instructions will be before or after closing arguments, but if before, (and Merchan covers FECA in Jury instructions) the defense can cover FECA in their closing.
Maybe this is why Merchan needs an 8 day break to consult the powers that be to find a way through this.