Audio problem: At the end, when Fitch is talking with both guys, he laughs, but from the side shot, his mouth is not moving. Seconds later, after he talks about the 110 million dollars, his mouth is not moving in sync with what he's saying.
Deliberate mistake: The investigator on his way to Garner, IN, is actually driving along Interstate I-10 somewhere between Louisiana and Florida. The shot of the freeway sign for "Gardner" is from off I-10 in the Florida panhandle and represents Gardner, Florida. The scene shot in Gardner, IN in front of Rachel Weiss' mother's home depicts the Crescent City Connection bridge in New Orleans. This scene is actually shot in Gretna, LA, on the West Bank of the Mississippi, just across from New Orleans.
Factual error: Nick Easter is being sequestered for the jury and comments that the Sherriff's office is across the street from the Mall on Esplanade Ave. The Esplanade Mall is actually in Kenner, which is in Jefferson Parish, but has its own Police Dept. The Kenner PD is nowhere near the Mall. The Jefferson Parish Sherriff's office does not have a station in Kenner.
Other mistake: When Marley calls Finch after her fight in the apartment, Finch slams the phone down 3 times. The plug for the handset flies up, but the cord was never tight enough to get pulled from the phone.
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.