The crossing of diocesan boundaries without permission or invitation is an offence, the Windsor Report claims, ‘not only against traditional and oft-repeated Anglican practice [alluding, no doubt to Lambeth 1988 and 1998] but also against some of the longest standing regulations of the early and undivided church (Canon 8 of Nicaea)’
N.T. (‘Tom’) Wright, Bishop of Durham and one of the members of the Commission, has explained what he takes to be the significance of the Canon. Says Wright: ‘I think not a lot of people know this, but it’s important to say this was a question that the early fathers faced at the same time as they were hammering out the doctrine of the person of Jesus Christ, and that they gave it their time to say people should not do this because that’s not how episcopacy works.’
Clergy will by now have run to their copies of the Canons of Nicaea. And they will have realised that Canon 8 says nothing whatever about bishops ‘crossing boundaries’. It is about ‘clergy flight’ and ‘clergy poaching’; about priests who seek to flee discipline in one jurisdiction by taking refuge in another, and about bishops who seek to lure clergy from another diocese to work in their own. (The general area now dealt with by Canons 265-272 of the present Codex Iuris Canonici)
It is not easy to see how all this helps Bishop Wright and his fellow Commissioners. (though one can see perfectly well why they wish it did!).
When it comes to actual ‘crossings of boundaries’ by particular bishops in the period immediately after Nicaea, however, things are very much clearer.
Sozomen in his Ecclesiastical History makes it quite clear that Athanasius performed ordinations in the dioceses of Arians or semi-Arians. Theodoret of Cyrrhus gives an account of the activities of Eusebius of Samosata (an Athanasian side-kick) and incidentally names five bishops Eusebius had consecrated. Epiphanius of Salamis (famous for his robust castigation of those who proposed the ordination of women) did the same in his native Palestine. Not until well after Chalcedon  did this principled ‘boundary crossing’ cease.
If, contrary to its plain meaning, Canon 8 of Nicaea did forbid ‘boundary crossing’ by bishops in defence of orthodoxy, it was a notion which, in current Anglican parlance, was not ‘received’.