John Mozeliak over-extended himself on three arbitration extensions during his tenure. The contracts given to outfielder Stephen Piscotty, first baseman Allen Craig, and outfielder/first baseman Jose Martinez went sour on Mozeliak very quickly.
Stephen Piscotty didn't exceed 5.0 fWAR during his contract, Allen Craig was a flat zero during his contract, and Jose Martinez was actually worth negative fWAR when his extension was all said and done. Stephen Piscotty signed a deal in April of 2017 after just one full professional season. The contract bought out his remaining pre-arb and arb years in addition to one year of free agency.
Piscotty's first year and a half in the league before his extension were strong, but injuries and underperformance limited his ability to reach his ceiling. After the Cardinals traded him in the winter of 2017, Piscotty was able to be closer to his mother who had ALS. Piscotty's first season in Oakland was great once again, but his mother passed away in May of 2018, and he wasn't able to rebound on the field.
Allen Craig was a late-round draft pick for the Cardinals. He was one of the heroes of the 2011 World Series run, and he had three strong offensive seasons between 2011 and 2013. Craig signed his deal in March of 2013 for five years and $31 million. His first year under the contract was good, as he racked up 2.5 fWAR. His next two seasons across St. Louis and Boston were both negative fWAR seasons for the first baseman/outfielder. Craig's heroics in the 2011 World Series won't be forgotten, but his contract extension soured very quickly.
Jose Martinez was the final extension during John Mozeliak's run of contracts to arbitration and pre-arbitration players. He signed a two-year, $3.5 million deal in February of 2019 to buy out his final year of arbitration and one year of free agency. Martinez wasn't known for his defense, and he didn't have a true position anywhere on the field, but the hope was that his offense would outweigh his defensive shortcomings. The relative cheapness of his contract also made it low risk.
Martinez's defensive inabilities were still present in 2019 and 2020, but his offense also plummeted. Eventually, Martinez was traded to the Tampa Bay Rays with Randy Arozarena for pitching prospect Matthew Liberatore.