When one compares John 5:3-4 in the KJV with many modern translations, one will notice that much of v.3 is missing and v.4 is absent in translations such as the NRSV:
In these lay a great multitude of impotent folk of blind, halt, withered, waiting for the moving of the water. For an angel went down at a certain season into the pool, and troubled the water: whosoever then first after the troubling of the water stepped in was made whole of whatsoever disease he had. (KJV)
In these lay many invalids--blind, lame, and paralyzed. (NRSV)
It is universally accepted by New Testament textual critics that the longer reading is a later interpolation into the text. Writing on this, Philip Comfort, a conservative Protestant scholar, wrote the following in a volume I *highly* recommend:
This portion (5:3b-4) was probably not written by John, because it is not found in the earliest manuscripts (P66 P75 א B C* T), and where it does occur in later manuscripts it is often marked with obeli (marks like asterisks) to signal spuriousness (so Π 047 Syrh marking 5:4). The passage was a later addition--even added to manuscripts, such as A and C, that did not originally contain the portion. This scribal gloss is characteristic of the expansions that occurred in gospel texts after the fourth century. The expansion happened in two phases: First came the addition of 5:3b--inserted to explain what the sick people were waiting for; and then came 5:4--inserted to provide an explanation about the troubling of the water mentioned in 5:7. Of course, the second expansion is fuller and more imaginative. Nearly all modern textual critics and translators will not accept the longer portion as part of the original text. NASB and HCSB however continue to retain verses in deference to the KJV tradition. (Philip W. Comfort, New Testament Text and Translation Commentary: Commentary on the variant readings of the ancient New Testament manuscripts and how they related to the major English translations [Carol Stream, Illin.: Tyndale House Publishers, 2008], 273)
The footnote to this passage in the NET offers the following comment on this textual variation:
The majority of later MSS (C3 Θ Ψ 078 ƒ1, 13 Û) add the following to Joh 5:3: "waiting for the moving of the water. Joh 5:4 For an angel of the Lord went down and stirred up the water at certain times. Whoever first stepped in after the stirring of the water was healed from whatever disease which he suffered." Other MSS include only v. Joh 5:3 (Ac D 33 lat) or v. Joh 5:4 (A L it). Few textual scholars today would accept the authenticity of any portion of vv. Joh 5:3-4, for they are not found in the earliest and best witnesses (î66, 75 א B C* T pc co), they include un-Johannine vocabulary and syntax, several of the MSS that include the verses mark them as spurious (with an asterisk or obelisk), and because there is a great amount of textual diversity among the witnesses that do include the verses. The present translation follows NA27 in omitting the verse number, a procedure also followed by a number of other modern translations.