State v. Ryan C. Diehl, 941 N.W.2d 272

Police stopped Ryan Diehl for expired registration; dispatch showed he was subject to a .02 prohibited alcohol concentration (PAC) because of multiple prior OWI convictions.
Diehl stipulated to his prior OWI convictions pretrial so the jury would only decide (1) whether he drove and (2) whether his BAC exceeded .02 at the time.
At trial the prosecutor repeatedly emphasized Diehl’s .02 restriction and, on cross, asked if Diehl had prior convictions; Diehl replied “Yeah. Twice.” Defense counsel made no objections.
Diehl presented an expert who opined his BAC at the time of the stop was likely below .02; the State did not present an absorption analysis of its own.
Jury convicted Diehl for operating with a prohibited alcohol concentration; on appeal Diehl argued trial counsel was ineffective for failing to object to irrelevant, unfairly prejudicial questioning that invited a propensity inference.
The Court of Appeals reversed and remanded for a new trial, finding the questions irrelevant/prejudicial and that counsel’s failure to object was deficient and prejudicial under Strickland.

Issue	State's Argument	Diehl's Argument	Held
Admissibility of prosecutor’s questioning that invited inference of prior OWIs	Questions provided necessary context and impeachment; did not explicitly ask about OWI convictions	Questions were irrelevant to the elements and created impermissible propensity inference under Alexander	The questions were irrelevant and unfairly prejudicial; should have been excluded under Alexander and Wisconsin evidence rules
Ineffective assistance for failing to object	Counsel may have had strategic reasons; presumption of strategy	Failing to object to repetitive, prejudicial questioning was objectively unreasonable	Counsel’s performance was deficient for not objecting or seeking a sidebar once questioning moved into dangerous territory
Prejudice (Strickland prejudice prong) — would result likely differ?	Evidence and expert weaknesses made conviction likely regardless; any error harmless	Case was close: expert provided a plausible theory BAC < .02; prejudice could have tipped jury	Prejudice shown: reasonable probability of different result absent the error; new trial required
Use of prior-conviction questioning as impeachment	Prosecutor may impeach by asking number of prior convictions (permitted) and did not ask about OWI specifics	Context made the impeachment question imply OWI priors and thus risked unfair prejudice	Even if impeachment questions are sometimes permissible, here context created an impermissible inference and objection should have been sustained

State v. Alexander, 214 Wis. 2d 628, 571 N.W.2d 662 (1997) (when defendant stipulates to prior OWIs, evidence of those priors is inadmissible because probative value is substantially outweighed by unfair prejudice)
State v. Warbelton, 315 Wis. 2d 253, 759 N.W.2d 557 (2009) (explaining the acute prejudice and stigma from prior OWI evidence in repeat OWI prosecutions)
Strickland v. Washington, 466 U.S. 668 (1984) (two-part ineffective-assistance standard: deficient performance and prejudice)
Old Chief v. United States, 519 U.S. 172 (1997) (prosecution’s storytelling need does not justify admitting prejudicial prior-conviction evidence when defendant stipulates)
State v. Machner, 92 Wis. 2d 797, 285 N.W.2d 905 (Ct. App. 1979) (procedure for developing record on ineffective-assistance claims)