"not seen" KJV/NKJV
The difference here is textual rather than translational (which is the case most of the time that the KJV/NKJV stand against other modern versions in the NT). Does it affect our understanding of the text?
The easiest way to understand this verse is to look at this "middle of the road" way of translating it, courtesy of the NET:
"Let no one who delights in humility and the worship of angels pass judgment on you. That person goes on at great lengths about what he has supposedly seen, but he is puffed up with empty notions by his fleshly mind."
We are told such a man claims to see certain things, but he has not really seen them, because they are creations of his own mind. To say he HAS seen them is merely to repeat his claim, to say he HAS NOT seen them is to point out reality. There is no conflict expressing it both ways.
As regards the textual difference, the earlier manuscripts leave out "not," while the majority of manuscripts have it. However, of those that have it, two of them apparently use a different Greek form of "not," lending more weight to the idea that it is a later addition.
1 Peter 3:3
[Do not let your adornment be outward] - KJV
[Do not let your adornment be merely outward] - Many others
The Greek word for adornment is "kosmos", which is used elsewhere in Scripture to refer to the system of this present world. In relation to personal appearance, it takes on the concept of arrangement, a person's specific arrangement of clothes, makeup, jewelry, and whatever else is placed on the body. Adornment, as used here, is NOT referring to clothes directly.
This difference is entirely translational. The difference between "not ... outward" and "not ... merely outward" is one of obviousness; which is to say that obviously you WILL have some arrangement of clothes, etc. on your body--everyone does. So then "not ... merely outward" is an exhortation to ALSO focus inward on the spirit.
"Not outward" says it differently; your focus should be on your spirit rather than on your fashion.
I like "not outward" better because it fits the thrust of the message more directly. However it's important to recognize two things here: saying it both ways doesn't create a contradiction, and "not outward" also finds a home in several modern versions, such as the NIV and NRSV.
"I live" - KJV
"I no longer live" - NIV
This certainly looks damning until you read the entire verse in each:
"I have been crucified with Christ and I no longer live, but Christ lives in me. The life I live in the body, I live by faith in the Son of God, who loved me and gave himself for me." NIV
"I am crucified with Christ: nevertheless I live ; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me." KJV
The difference is purely translational. Incidentally, the NIV is more formal/literal here than the KJV, IMO.
"He might know your estate" - KJV
"You may know about our circumstances" - NIV
The most important to thing to note here is that verse 8 in the KJV contradicts the entire purpose of Tychicus' visit as expressed in the verses around it:
v.7a KJV - "All my state shall Tychicus declare unto you"
v.9b KJV - "They shall make known unto you all things which are done here."
Lest you, dear reader, think that v.8 in the KJV is Paul stating that Tychicus was ALSO to find out how the Colossians were doing, and not a contradiction after all, I present the entire verse:
"Whom I have sent unto you for the same purpose, that he might know your estate, and comfort your hearts;"
"same purpose" is referring to "that he might know your estate," which is a contradiction since "same purpose" also points back to v.7, "all my state shall Tychicus declare unto you."
This verse brings up some very interesting observations regarding the battle between critical and majority text advocates. The KJV reading here is not merely the reading of the Textus Receptus, it is the reading of the Majority Text, and furthermore, it is the reading of the Byzantine MS tradition at large. However, such a reading is an obvious, unambiguous error. The KJV and the Byzantine/Majority text are flat-out wrong here.
The fact that the NA/UBS critical text reading is the obvious right choice here is a unique example of the value of modern, eclectic textual criticism.
A vast majority of KJVO "defense" material is merely chart after chart comparing select words in verses, such the above. So zealous are these people to demonstrate a scandalous difference that they end up listing the most petty and worthless "differences."