Yesterday I had a King County District Court judge declare a mistrial after a trooper testified using language specifically excluded during motions in limine. No matter the intent (whether it was ill-will, an overzealous trooper or unintentional testimony) using language specifically precluded during pre-trial motions or motions in limine which materially affect a defendant's right to a fair trial is grounds for a mistrial.
Here the Trooper had been instructed not to use language which was verbatim from the DUI statute and the "to convict" jury instruction. The trooper specifically uttered the words "affected by" when describing his opinion of my client's alcohol consumption. The court had previously admonished him on another statement he made in near violation of motions in limine regarding one of the field sobriety tests. His cumulative testimony with respect to the motions in limine is what resulted in the mistrial.
It is a prosecutor's role to ensure that a defendant have a fair trial, not to seek as many convictions as they can. When a witness usurps the prosecutor's role, ultimately the prosecutor is responsible as the trooper, in this case, is an agent of the state/government. The court made the correct decision.
Mistrials happen, witnesses sometimes say things they are not supposed to say, but a prosecutor must ensure that their witness has been properly advised of the court's prior rulings so that taxpayer money and resources on a new trial are not wasted. The court further instructed that the State should take into consideration the Trooper's clear violation (and conduct during testimony) of the court's prior order against specific testimony. Hopefully the State will take that into consideration, but given the county and prosecuting agency in which this occured it is highly doubtful.