I researched it, and I found what I had forgotten. The man was Gregory of Nazianzus, Archbishop of Constantinople.
The canon broken was:
Canon 15
On account of the great disturbance and discords that occur, it is decreed that the custom prevailing in certain places contrary to the Canon, must wholly be done away; so that neither bishop, elder, nor deacon shall pass from city to city. And if anyone, after this decree of the holy and great Synod, shall attempt any such thing, or continue in any such course, his proceedings shall be utterly void, and he shall be restored to the Church for which he was ordained bishop or elder.
He had been moved from the rather obscure see of Sasima to become Bishop of Constantinople. He replaced an Arian. Lots of drama was involved in it. When the man who had been expected to preside at the First Council of Constantinople died, Gregory was expected to take over; but people opposed him because of how he became Bishop. He was sick and decided to resign.
Then things got even stranger.
Nectarius of Constantinople - Wikipedia
When the Emperor Theodosius I wanted the Bishops at the Council to suggest new candidates and reserving to himself the right of making the choice, the Bishop of Antioch put at the bottom of his list, Nectarius' name. The Emperor having read the lists, declared Nectarius to be his choice. This caused some amazement amongst the Fathers who wanted to know - who and what was this Nectarius? He was still only a catechumen. There was much astonishment at the emperor's unexpected choice, but the people of Constantinople were delighted at the news as was the whole council.
Nectarius was duly baptized and his clothes were changed for the robes of a Bishop of the Imperial city and became at once president of the Second Ecumenical Council.
Someone who hadn't been baptized one day became Bishop the next and presided over this Ecumenical Council. Strange that no one objected to Nectarius since his promotion was also a violation of the canons of Nicea.
Canon 2
Forasmuch as, either from necessity, or through the urgency of individuals, many things have been done contrary to the Ecclesiastical canon, so that men just converted from heathenism to the faith, and who have been instructed but a little while, are straightway brought to the spiritual laver, and as soon as they have been baptized, are advanced to the episcopate [office of bishop] or the presbyterate [office of elder], it has seemed right to us that for the time to come no such thing shall be done. For to the catechumen himself there is need of time and of a longer trial after baptism. For the apostolical saying is clear, "Not a novice; lest, being lifted up with pride, he fall into condemnation and the snare of the devil." But if, as time goes on, any sensual [lit., "soulish"] sin should be found out about the person, and he should be convicted by two or three witnesses, let him cease from the clerical office. And whoso shall transgress these [enactments] will imperil his own clerical position, as a person who presumes to disobey the great Synod.
It also violated the canon about how a bishop was to be chosen. The Emperor isn't supposed to be making decisions like that.
Canon 4
It is by all means proper that a bishop should be appointed by all the bishops in the province; but should this be difficult, either on account of urgent necessity or because of distance, three at least should meet together, and the suffrages of the absent [bishops] also being given and communicated in writing, then the ordination should take place.† But in every province the ratification of what is done should be left to the Metropolitan.
We can see how so few people then took the Council of Nicea seriously. If people didn't like Gregory, they'd object to how he was promoted saying correctly how it was uncanonical; but if the Emperor wanted Nectarius, they could overlook how he was made a Bishop.
The Emperor got what he wanted. I must ask then, why should I take these councils seriously when the people at them didn't? If it suited them, they'd cite them. If it didn't suit them, they'd ignore them.