NIV FOOTNOTES
[1]Mat_17:20 Some manuscripts you. 21 But this kind does not go out except by prayer and fasting.
I just wanted to show that it is the NIV. (at least in the footnotes)
Also here is a note from the NET Bible as to why the verse is not in it...
39tc Many important mss (א* B Θ 0281 33 579 892* pc e ff1 sys,c sa) do not include 17:21 “But this kind does not go out except by prayer and fasting.” The verse is included in א2 C D L W Ë1,13 Ï lat, but is almost certainly not original. As B. M. Metzger notes, “Since there is no satisfactory reason why the passage, if originally present in Matthew, should have been omitted in a wide variety of witnesses, and since copyists frequently inserted material derived from another Gospel, it appears that most manuscripts have been assimilated to the parallel in Mk 9.29” (TCGNT 35). The present translation follows NA27 in omitting the verse number as well, a procedure also followed by a number of other modern translations.
Then here is the note on Mark 9.29
48tc Most witnesses, even early and excellent ones (Ì45vid א2 A C D L W Θ Ψ Ë1,13 33 Ï lat co), have “and fasting” (καὶ νηστείᾳ, kai nhsteia) after “prayer” here. But this seems to be a motivated reading, due to the early church’s emphasis on fasting (TCGNT 85; cf., e.g., 2 Clem. 16:4; Pol. Phil 7:2; Did. 1:3; 7:4). That the most important witnesses (א* B), as well as a few others (0274 2427 k), lack καὶ νηστείᾳ, when a good reason for the omission is difficult to find, argues strongly for the shorter reading.
Now with that said...
I believe "and fasting" should be in Mark 9:29...
But not in the Matthew passage...
Here is my reasoning...
Yes scribes would have tried to make parallel passages read the same.
So they would have inserted the phrase,
"this kind does not go out except by prayer and fasting."
To make it read the same as Mark 9...
Of course that brings us to Mark 9..."and fasting"
If they wanted to make Matt read the same as Mark, the "and fasting" would have had to be in Mark.
Also.. the notes in the NET Bible clearly says that, "Most witnesses, even early and excellent ones" have "and fasting"
So to sum this up... I believe the NIV is right in Matthew 17:21, and wrong in Mark 9:29