Megillah, Chapter 4, Mishnah 8. This mishnah deals with visible practices that arouse suspicion that the person performing them is inclined toward the ways of heresy.
Two declarations that disqualify a chazzan:
"Ha'omer: eini over lifnei ha'teivah be'tzevu'in" - one who declares that he is unwilling to go down before the amud and serve as chazzan in colored garments, but only in white ones. This insistence is not a halachic requirement at all, and therefore the mishnah rules: "af be'levanah lo ya'avor" - even in the white garments, which he is willing to wear, we do not allow him to pass before the ark. The reason: in that era there were heretics who made a point of wearing specifically white, and we must suspect that a trace of heresy has attached itself to him.
"Be'sandal eini over" - one who says that he is unwilling to pass before the ark while wearing a sandal is likewise disqualified: "af yachef lo ya'avor". Here too the concern is heresy, for there were heretics at that time who made a point of not wearing shoes.
"Ha'oseh tefilaso agulah - sakanah v'ein bah mitzvah":
Danger - one who makes his tefillin round instead of square: if he strikes against something, the tefillin are liable to wound his head more easily than square, flat tefillin would.
And there is no mitzvah in it - it is a halachah given to Moshe at Sinai that tefillin must be square, and one who does not fulfill their proper form fulfills no mitzvah at all.
Placing them in the wrong spot and an improper covering:
"Nesanah al mitzcho o al pas yado - harei zo derech ha'minus" - one who places the head tefillin in the middle of the forehead rather than on the head itself, above the hairline, or the hand tefillin on the palm rather than on the muscle - this is the way of the minim, who were a type of heretic.
"Tzipan zahav u'nesanan al beis unkeli - harei zo derech ha'chitzonim" - one who overlays his tefillin with gold instead of having them black, and one who places them on top of his sleeve rather than on his flesh - this is the way of the chitzonim, those people who followed their own understanding rather than the understanding of Chazal. This is a form of heresy, and if one sees a person behaving this way, it is a sign of an inclination toward the ways of the heretics.
In summary: in this mishnah we learned about outward practices that arouse suspicion of heresy: a declared refusal to wear colored garments or to wear a sandal during prayer - which disqualifies the one who says it from passing before the ark even in the manner he is willing to accept; making tefillin round - which is dangerous and fulfills no mitzvah; and placing them in the wrong spot, overlaying them with gold, or placing them on top of one's clothing - which are the way of the minim and the chitzonim.