Citing R. Israel Baal Shem:

“Beware of their gachelet (glowing coal) lest you be burnt . . . all their words are like fiery gachalot (coals).” (Avot 2:10) This is difficult to understand: if the unqualified term gachelet implies burning [coals; embers], why then qualify it [in the conclusion as] “fiery coals”? If again, gachelet is defined to relate to omemot, i.e., dimmed (dying coals), why would you need to be careful “lest you be burnt” as gachelet simply refers to omemot?

[The Baal Shem Tov], of blessed memory, thus said:

A perfect tzadik (saint) may sometimes fall from his level and worship God in a mode of katnut (constricted consciousness): 1 he does not pray with great kavanah (intention), and sometimes may even go idle. 2 Another person seeing the tzadik in that state of not praying or studying with great kavanah, and sometimes going idle, may very well think to himself that he can act likewise. After all, if the saintly and pious can do so, how much more so he himself! The teacher [of our Mishnah] thus cautions: “Do not compare yourself to the Torah-scholar and tzadik! For when the tzadik will awaken from his ‘sleep’ 3 and again prays and studies as he used to, he will elevate all his idle words [or deeds]. 4 You, who observed him, however, you are but a simple person who is totally unaware of the mystery of Divine worship. How dare you, then, compare yourself to him!”

This is the meaning of “beware of their glowing coals”:

Even when [tzadikim] have fallen from their level and are like “dimmed coals” for uttering idle words or involved with idle deeds, beware! Do not apply a lesson from them [for yourself], for even their idle talk is like fiery coals, as stated above. 5