Kommentar zu Megillah 4:8

אף בלבנים לא יעבור – for we suspect lest heresy was cast on him, for idolaters are strict in this.

If one says, “I will not pass before the ark in colored clothes,” even in white clothes he may not pass before it.
[If one says], “I will not pass before it in shoes,” even barefoot he may not pass before it.
One who makes his tefillin [for the head] round, it is dangerous and has no religious value.
If he put them on his forehead or on the palm of his hand, behold this is the way of heresy.
If he overlaid them with gold or put [the one for the hand] on his sleeve, behold this is the manner of the outsiders.
The next two mishnayot deal with certain practices which the rabbis deemed to be heretical or at least potentially heretical.
Sections one or two: In the first two sections we learn of people who refuse to pass before the ark (to lead the Amidah) either while wearing colored robes or while wearing shoes. The rabbis suspected that one who demanded to wear white clothes or go barefoot may have had heretical beliefs. Therefore, they said that such a person cannot pass before the ark at all, even in white clothes or barefoot. In other words, wearing white clothes and going barefoot seem to have been valid practices but one who insists upon them is suspected of heresy.
We should note that the groups being described here seem to be taking Temple practice and applying it to the synagogue. In the Temple the priests’ robes were white and they went barefoot. The mishnah may be trying to emphasize that the synagogue is not the Temple and one who insists on dressing in the synagogue as if it were the Temple is potentially a heretic. There also may be a covert battle for leadership in this mishnah between priests and rabbis. Rabbis may be telling priests that when in the synagogue leading the Amidah (as opposed to reciting the priestly blessing) they are functioning as regular Jews and not as priests.
Section three: The boxes of tefillin are supposed to be square. Our mishnah deals with a period of oppression when the Romans prohibited Jews from wearing tefillin. In response someone makes his tefillin round so that the Romans will not notice that he is wearing tefillin. According to the mishnah this attempt is doubly mistaken. The Romans will realize that he is wearing tefillin and therefore it is still dangerous. Secondly, by making his tefillin round he is not fulfilling the mitzvah of tefillin. There is also the idea that tefillin can protect a person from danger. But since these tefillin are not valid they offer no protection from the Roman oppressors.
Section four: The Torah says that you should place tefillin “as a sign upon your hand and as a remembrance between your eyes.” Non-rabbinic groups of Jews (sectarians) interpreted these verses literally; tefillin are put on one’s hands and on the forehead between one’s eyes. The rabbis did not interpret the verses literally tefillin go on top of one’s head, where the hairline ends, and on one’s arms, next to one’s heart. A person who wears his tefillin between the eyes or on the hand is acting as a heretic. I should note that I have seen many, many instances of people wearing their tefillin to low on their heads. One who wears tefillin between his eyes has not fulfilled his obligation.
Section five: Covering tefillin in gold or wearing them on one’s sleeves is not proper fulfillment of the mitzvah. The mishnah deems this as the practice of “outsiders” those who have separated from the rabbinic fold.

העושה תפילין שבראשו עגולות – like a nut or an egg

סכנה – it should enter on his head

ואין בה מצוה – as square (i.e. quadrilateral) phylacteries are a usage dating from Moses as delivered from Sinai (i.e. a traditional law or traditional interpretation of a written law).

הרי זה דרך מינות – as they despite the words of the Sages and follow after the [literal] implication of the Bible as they say: “between your eyes” (Deuteronomy 6:8) literally, “on your hand,” (ibid.,) literally. But the Sages learned through an analogy [that the words] “between your eyes” is in the place of one’s hair[line] on one’s head, the place where the baby’s brain pulsates (see Talmud Menahot 37a). [The words] “on your hand” [means] at the height of the arm, on the biceps muscle (Talmud Menahot 37a) at the top of the arm – opposite the heart.

ציפן זהב וכו' – And the Biblical verse states (Exodus 13:9): “[And this shall serve as a sign on your head and as a reminder on your forehead] – in order that the Teaching of the LORD may be in your mouth….], what is permitted [to be placed] in your mouth, so that everything will come from pure cattle and not from gold.

ונתנה על בית אונקלי – on the sleeve of clothing on the outside, and the Biblical verse states: “it shall serve for YOU as a sign,” and not as a sign for others.

הרי זה דרך החיצונים – people who follow after their own thoughts, outside of the opinions of the Sages.