Silanus's defeat (109-108 BC) was perhaps the most obscure Roman defeat during the Cimbric War with both its location and date in doubt, and even one source making it a Roman victory!
The Cimbri first appeared in Roman sources in 113 BC, when they defeated the consul Cn Papirius Carbo near the city of Noreia, to the north-east of the Alps. After that victory they moved off into Gaul, and disappeared from the records for several years.
The Periochae of Livy places the battle in 109 BC, and says that the 'Consul Marcus Junius Silanus unsuccessfully fought against the Cimbrians'. The Cimbri sent envoys to the Senate to demand land, but they were ignored.
Velleius Paterculus mentions the battle in passing, as a previous victory won by the Cimbri and Teutons over Roman armies.
Florus begins his account of the Cimbric War with this battle. This time the Cimbri, having been forced out of Gaul and Spain, entered Italy, and sent representatives to Silanus, asking for land in return for military service. The Senate turned down this offer, and the Cimbri then attacked Silanus, defeating him.
It seems unlikely that this battle took place on the Italian side of the Alps, as that would have made it a major defeat and worthy of far more note than it gets. We also hear nothing about the size of the Roman army, so have no idea how serious a defeat it actually was.
Cn. Domitius, one of the tribunes of 104, unsuccessfully attempted to prosecutre Silanus because of the injuries suffered by Egritomarus, one of his father's friends, but Silanus was acquitted.