A peson steals a car in Anderson County and is subsequently arrested in Smith County by DPS troopers. Officers go to the arrest location, get the suspect for UUMV and transport him to the Anderson County Jail. The officers fail to obtain a magistrate's warning prior to transporting suspect from Smith County back to Anderson County. How does this effect the arrest and future prosecution of the defendant? What happens to any statement made by the defendant in Anderson County as a result of no such warning?

There may be no effect. There was a 2005 amendment to CCP Art. 14.06 allowing a prisoner arrested in one county to be magistrated in another county to provide more expeditious warnings.

Did you get him magistrated once he got back to Anderson County prior to the statement? If so, you're probably in good shape.