UK Apologetics Reply:
The answer to your question depends which translation you are using. Mostly this does not happen too much unless you are using something like The Amplified Bible, which does not claim to be a literal translation but, rather, it is a paraphrase. Things are purposely added to the text to help broaden and expand the meaning.
One may think that this is not good, but this can greatly assist people who are newer in the Faith until their understanding deepens - as long the 'Amplified' does not become their primary version.
But, to return to the more usual translations, Ephesians 2:11 may be the sort of thing you are talking about. Both the KJV and NIV put a part of that verse into brackets - interestingly, they put different parts into brackets! The translator has the right to do that if it is felt that to do so will enable the verse to 'hang together' a bit better in English.
First of all, the Greek has it:
dio mnhmoneuete oti umeis pote ta eqnh en sarki oi legomenoi akrobustia upo thV legomenhV peritomhV en sarki ceiropoihtou
(if you have the Greek 'symbol' font installed on your computer - and you should have - you will see the actual Greek letters, not a dreadful transliteration. Unfortunately, Firefox never seems to show the actual Greek letters, neither will Safari or Opera. Internet Explorer 6-8 and Google Chrome are excellent in this regard).
The KJV renders this:
2:11: Wherefore remember, that ye [being] in time past Gentiles in the flesh, who are called Uncircumcision by that which is called the Circumcision in the flesh made by hands;
And the NIV has it:
Therefore, remember that formerly you who are Gentiles by birth and called "uncircumcised" by those who call themselves "the circumcision" (that done in the body by the hands of men)-
The World English Bible has it:
Therefore remember that once you, the Gentiles in the flesh, who are called 'uncircumcision' by that which is called 'circumcision,' (in the flesh, made by hands);
So three translations (out of about fourteen that I checked), felt it necessary to add just a little something [parentheses] to the original Greek text in order to make the verse more understandable. I think we can see why - this is quite a tricky verse!
In addition, the KJV also felt it necessary to use capital letters for the the two opening letters of the words Uncircumcision and Circumcision (only small letters in the Greek), while the NIV simply puts those words into inverted commas. Why is this? Undoubtedly because those two simple words only really refer to a physical practice, but when Paul uses them, he is often referring to a whole Judaistic legalistic movement, so something extra in the way of emphasis needs to be placed on those two words.
So such devices have sometimes been used by translators, although, I don't think too often. Also, in the case of the Apostle Paul, he often wrote in very long sentences which lend themselves to having part of the sentence bracketed off in order for it to 'hang together' a bit better in English. And I think that is your answer.
Robin A. Brace, December 15th 2009.