If you have an SATB piece of music that begins with, say, "DFAD," and ends with "GBDG" (picardy third in this case), and if this piece is repeated (e.g., stanzas in a hymn), would the tenor be committing the "error" of singing an augmented fourth? (This question, of course, applies to any of the other "rules" of part-writing.) It seems like it would be indisputable if it occurred in the middle of the piece, but are normal part-writing rules in effect when repeating the singing as much as continuing the singing?
I'm going to take a back route to this answer.
In Schenkerian theory there is a concept called interruption. One common form of interruption has scale-degree 2 in the melody above a root-position V chord; this scale-degree 2 moves to scale-degree 5 above a root-position I chord. This results in a motion of:
2 5 V I
Or, in C major:
D G G C
In other words, parallel perfect fifths.
You're welcome to read more about interruption if you like, but what's important for this answer is that there are times, typically at ends of phrases, where these parallel fifths occur. I won't go into Schenker's reasons here, but the point is that these part-writing rules do not apply at this stanza break, nor do they apply at various phrase endings, points of repetition, etc.