Plot hole: Fitch should have been aware of Herman Grimes being a selected juror for it was revealed that everyone who received a jury summons for the case was photographed in the beginning of the movie. Herman would have received a summons just the same as the rest and would have been treated the same.
Plot hole: When the lunch was late, wouldn't the bailiff have been wondering why and checking with the restaurant? She knew that the jury was in the room waiting.
Plot hole: The whole point of Marley and Nick's plan was to get back at Fitch, it's also revealed they have been doing this for a while in different cities. There is no way they could know what cases Fitch would take.
Chosen answer: Hackman's team was closely monitoring what Hoffman was doing in regards to the tampering, as evidenced by their taping of his conversation with Weiss when he decides not to pay. Cusack and Weiss probably anticipated this so were forced to offer the deal to Hoffman to keep up the illusion.