Two important, shorter Byzantine readings in 1 John

While the minuscules are not unified here, there is still a clear Byz text identified by the ECM. Because of this unity, you will not find this variant in Robinson-Pierpont as a Byzantine variant though it is in the apparatus as an NA27 reading. The obvious explanation for the second reading is, of course, homoiteleuton (ἔχει ... ἔχει).

By way of illustration, here is the correction of the text in 424 adding the text back in followed by a second correction expunging it.

The second such omission is just a few verses later in 1 John 3.1. There the main evidence is

Once again, we have the earliest evidence, several dozen minuscules, and most of the versions in favor of the longer reading and the Byzantine manuscripts in favor of the shorter. This variant won’t show up in the RP as an intra-Byzantine variant either. Again, the simplest explanation for the Byzantine reading is homoioteleuton, the eye skipping from -μεν to -μεν.

Klaus Wachtel (Der Bzyantinische Text, 302–303) also suggests that the shorter reading would be preferable because it removes the abrupt shift from subjunctive (κληθῶμεν) to indicative (ἐσμέν) following ἵνα. Confirming this as a possible motive is the fact that we find the subjunctive ὦμεν in 2544 and this appears to be what is translated by the Harklean Syriac and some Coptic witnesses. 

Here is this variant again in 424 showing another correction.

Both variants are pretty easy to deal with for reasoned and thoroughgoing eclectics and pretty difficult for Byzantine prioritists. It may be surprising to see the Byzantine tradition preserve such obvious mistakes, but in this, it also shows how careful the Byzantine scribes often were. It also suggests that, in some cases, the Byzantine text goes back to a single exemplar that is not the autograph and not one of our earliest Greek witnesses. These two cases also illustrate well the realty that no single text-type or manuscript has a corner on the original text all the time.