The Celtic occupation of Rome took place in 390 B.C. It may be one reason the Romans were so persistent in their aggressive attacks on the Celts over several centuries.
The Celtic occupation of Rome is instructive in many ways. It tells us much about the character of the Celts and their sense of honor and justice. Anyway, here it is in a nutshell.
In essence, a Celtic tribe, the Senones, crossed the Apennines in search of new land to settle. They camped outside the Etruscan city of Clusium and petitioned the city leaders for such land. The Etrusans were suspicious of these strangers and asked Rome for assistance (Rome having recently conquered the Etruscians). The Romans responded by sending ambassadors to talk to the Celts. The talks broke down and a battle ensued. Against all rules, the Roman ambassadors took up arms against the Celts and even killed a Celtic chieftain. The Celts were outraged and sent ambassadors to Rome to protest.
Although some Romans felt an apology was in order, most were not so inclined (due to the influence of the families of the Fabii brothers...the ones who were the ambassadors sent to parlay between the Etruscians and Celts). This enraged the Celts. Thus, they decided to march on Rome. On the way, they did not harm anyone or take anything. They met the Roman army outside Rome and defeated it. They then occupied the city for several months until an apology was issued along with some tribute. They then left of their own accord.
One can find other accounts of this event but most of those accounts are by those who had reason to hate or fear the Celts. In those accounts, the Celts are depicted as savages. However, more neutral accounts indicate the Celts were quite civilized and had a high sense of honor and justice.