The Bavli, Shabas 82 amud 1:
If before him [after defecating] were a pebble and grasses — [the proper course of action was a matter of dispute between two amoraim,] R. Chisda and R. Hamnuna. One [of them] said: one wipes with the pebble and does not wipe with the grasses. And [the other] said: one wipes with the grasses and does not wipe with the pebble.
They respond [to one of those propositions with a quotation from a tana]: "One who wipes with something that can catch fire — his lower teeth fall out." [So how can one of those two amoraim say to wipe with grasses?]
This is not a problem: [one was talking] about moist [grasses, whereas the other was talking] about dry ones.
Rashi has two very different explanations of this g'mara. My question is about his first explanation, which is this: When the amora is quoted as having said that "one wipes with the pebble and does not wipe with the grasses", Rashi explains that that's because the grasses are moist and cut the flesh.
Moist?? The end of the g'mara seems to say that moist ones are not a problem: dry ones are. Why does Rashi explain the first part of the g'mara as saying that moist ones are a problem; and if they are indeed a problem, then why does the end of the g'mara say that they are not?
Or does the g'mara mean that the amora who said to use grasses was referring to dry ones and the tana to moist ones? That'd reconcile the first Rashi, but would raise another question: if the problem is with flammable objects, then shouldn't moist grasses be less, not more, of a problem than dry ones?