What you're talking about is known as the "Seal of the Confessional" (sigillum confessionis), and it's taken very seriously. A violating priest is under threat of being excommunicated latae sententiae, meaning retroactively to the exact moment that he committed the offense.
The strict upholding of this doctrine goes back to at least 1151 to the Decretum Gratiani, a compilation of Canon Law compiled by a jurist name Gratian (as the name implies). The text is here, but unfortunately I couldn't find it in English quickly. Here's a relevant quote to summarize:
"Let the priest who dares to make known the sins of his penitent be deposed."
What is said to a priest during confession is not to be revealed to anyone, anytime, ever, even on threat of death.
I hope I'm not breaking sub rules by linking a current case that is relevant to answering this question, in which a priest is facing legal problems for refusing to speak about the content of a confession:
Source: Canon Law can be found here, relevant section being 983 ยง1
As for a famous case of priest violating The Seal, even a Google search isn't turning much up. Perhaps someone else can add something relevant.
I'd like to historicize your question a bit. I would argue that the issue of the seal of confession (ie, the confidentiality of confession by the priest not to be divulged under the pain of excommunication) revolves around two major pre-modern events: the so-called Gregorian Reforms (11-13th century) and the Catholic Reformation (16th century). Both events also are associated with a broader effort to improve the quality of the parish clergy (ie, those actually hearing confessions), through education, professionalization of clerical ranks and tighter oversight by bishops.
Annual confession became mandatory only in 1215, as decreed by the Fourth Lateran Council. That said, the movement toward the regularization of confession (and communion) starts about a century earlier. For example, the 12th century Gratian's Decretum, a medieval compendium of canon law, specifically states that any priest who dares to disclose the sins of the penitent, ought to be deposed. Offending priests, according to the decrees of the Fourth Lateran Council, were not only to be deposed, but confined to monasteries for a lifetime.
Starting in the 15th century, Jan Nepomucky a 14th century Bohemian priest (also known as St. John Nepomucene) became a kind of model of priestly confessional secrecy. According to his later vitae, while serving as the Abp of Prague, he was the confessor of Queen of Bohemia. The King, suspecting that his wife may have had a lover, pressed Jan to disclose any details of an affair, which the cleric promptly refused. For this, he was seized and thrown into the Vltava river, dying a martyr's death. Particularly in the sixteenth century, Jan Nepomucky became a kind of model of priestly virtue and a paragon of confessional secrecy.
As the Protestant Reformation swept across Europe in the sixteenth century, most Protestant reformers questioned not only the sacramental necessity and validity of auricular confession, they also criticized the moral laxness of the priests performing the said act. The Council of Trent (1545-63), echoing the proclamations of the Fourth Lateran, reaffirmed the validity of auricular confession, while calling for a higher intellectual as well as moral quality of the clergy performing it. Additionally, many local bishops initiated efforts to provide an environment which assured greater privacy between priest and penitent. Until the sixteenth century, there was no actual designated space for confession (usual space was a pew or a bench in rear of church). Carlo Borromeo, the Bishop of Milan was the first to pioneer the confessional box, which not only designated place for sacramental confession, it also ensured greater privacy, as well as actual anonymity for the penitent. In Baroque-era churches, paintings and statues of St. Jan Nepomucky as the patron saint of confession and the confessional seal, become very widespread (especially in Central-Eastern Europe), thus, despite medieval origins, the full utilization of his cult doesn't reach its peak until the 17-18th centuries.
I have done extensive research on episcopal visitations and tighter episcopal control over parish clergy between 16-18th centuries. In my region (Central-Eastern Europe), lay complaints to the bishop (or appointed episcopal visitor) on the breaking of the confessional seal were actually very rare; complaints about drunkenness, apathy, nepotism and financial laxity were far more frequent. In instances where such complaints did occur, it was as part of a larger package of alleged transgressions, so isolating an instance where only the seal of confession was broken simply didn't come up in my source base. When the said did occur, the accused priest was usually called before an episcopal court, and if found guilty, confined to the episcopal carcer (prison, dungeon) usually located somewhere beneath the cathedral.
Some literature: R.W. Swanson, Religion and Devotion in Europe, C.1215-c.1515 (Cambridge, UK: Cambridge University Press, 1995); David Myers, Poor Sinning Folks, Confession and Conscience in Counter-Reformation Germany (Ithaca, NY: Cornell University Press, 1996); Thomas N. Tentler, Sin and Confession on the Eve of the Reformation (Princeton: Princeton University Press, 1977), also an excellent article: John Bossy, The Social History of Confession in the Age of the Reformation in Transactions of the Royal Historical Society, Fifth Series, Vol. 25 (1975), pp. 21-38.
EDIT: silly typos!