I spent a good amount of time today looking through references where Robertson talks about postpositive conjunctions appearing in the wrong place:
(i) Postpositives. A number of words are always postpositive in Greek. In the N. T. ἄν, γάρ, γε, δέ, μέν, μέντοι, οὖν, τε never begin a sentence, in harmony with ancient Greek usage. These words commonly in the N. T. come in the second place, always so with μέντοι (Jo. 4:27, etc.). In the case of μέν the third place is occasionally found as 1 Pet. 2:4, the fourth as 2 Cor. 10:1, the fifth in Eph. 4:11; Jo. 16:22, or even the sixth in Jas. 3:17. It occupies the seventh place in Herm. Sim. viii, 5:1 (Mr. H. Scott has noted). In general these words vary in position according to the point to be made in relation to other words. So also οὖν is more commonly in the second, but varies to the third (Jo. 16:22) and fourth (1 Cor. 8:4). The same remark applies to γάρ, for which see Mk. 1:38; 2 Cor. 1:19. As to δέ, it may not only go to the fourth place (Jo. 8:16), but even appears in the fifth (1 Jo. 2:2), οὐ περὶ τῶν ἡμετέρων δέ. It stands in the sixth place in Test. XII. Patr. Judah, 9:1 (Mr. H. Scott reports).
A.T. Robertson, A Grammar of the Greek New Testament in the Light of Historical Research (Logos, 1919; 2006), 424.
Looking through the references, there were some interesting patterns:
Mark 1:38 (UBS4)
[FOC: εἰς τοῦτο] γὰρ ἐξῆλθον.
John 16:22 (UBS4)
22 [καὶ [TOP: ὑμεῖς] οὖν νῦν μὲν λύπην ἔχετε·]
1 Corinthians 8:4 (UBS4)
4 [Περὶ [FOC: τῆς βρώσεως]] οὖν [τῶν εἰδωλοθύτων], οἴδαμεν ὅτι οὐδὲν εἴδωλον ἐν κόσμῳ καὶ ὅτι οὐδεὶς θεὸς εἰ μὴ εἷς.
2 Corinthians 10:1 (UBS4)
[[TOP:ὃς] [FOC: κατὰ πρόσωπον] μὲν ταπεινὸς ἐν ὑμῖν]
1 Peter 2:4 (UBS4)
4 [[FOC: ὑπὸ ἀνθρώπων] μὲν ἀποδεδοκιμασμένον]
1 John 2:2 (UBS4)
2 καὶ αὐτὸς ἱλασμός ἐστιν περὶ τῶν ἁμαρτιῶν ἡμῶν, [οὐ [FOC: περὶ τῶν ἡμετέρων] δὲ [μόνον]]
I looked through some others too that I searched for. Not all of them are focused. Some are topics, but the majority are contrastive in either case. It seems that postpositives that are not in the right place are significant, which Robertson himself notes in passing in the quote above when he write, “In general these words vary in position according to the point to be made in relation to other words.”