[Mrs. Don-o]

I think the only proper circumstance would be imminent danger of death. And even then, if the person declares a "Catholic faith" in the Eucharist, he should be baptized first (it takes under 10 seconds, I timed it) --- and then given the precious Body and Blood of the Lord as Viaticum.

If he is already baptized (e.g. in a different Christian community which practices licit Baptism) and is in imminent danger of death, he should be received into the Catholic Church upon his sincere declaration of the Faith: and then, as above, Viaticum.

[piusv]

The 1917 Code of Canon Law is clear. There are no special circumstances:

It is forbidden to administer the sacraments of the Church to heretics or schismatics, even though they err in good faith and ask for them, unless they have first renounced their errors and been reconciled with the Church.

This must happen even if they are "in imminent danger of death". Note that it just isn't a declaration of faith in the Catholic Eucharist, but the renunciation of all of their errors and a full profession of all that the Catholic Church teaches.