Hosea 6:6: harmonising with NT destroys OT
Now I claim no expertise on Hosea and am looking at it only very superficially. But it seems clear that a new main section begins at chapter 4, with the Lord's charge against the Israelites that "there is no 'emet, no hesed, and no da`at of God in the land" (4:1). NIV translates these Hebrew words "faithfulness", "love" and "acknowledgement" respectively, and TNIV is unchanged. As Claude points out, it would be much more accurate and consistent to render the third word "knowledge" rather than "acknowledgement", especially as this is the key theme taken up in 4:6 where NIV and TNIV have "knowledge".
As the discourse continues, in 6:4 the Lord comes back to hesed: "Your hesed is like the morning mist, like the early dew that disappears". Here NIV and TNIV have "love", as in 4:1. In most cases that would be a weak translation of hesed, which implies loyalty and faithfulness as well as love, but the point here is of course that the loyalty and faithfulness aspects are absent.
So at 6:6 the Lord starts to sum up the section with a clear reference back to the charge in 4:1, perhaps an inclusio to mark the end of the section: "For I desire hesed, not sacrifice, and da`at of God rather than burnt offerings". But it is here that NIV and TNIV commit what I can only call a gross translation error. Rendering da`at as "acknowledgement" is at least consistent with 4:1. But here the translation of hesed is suddenly not "love" but "mercy".
Why? Is this a simple slip of the pen? I doubt it, for if so surely the TNIV translators would have been told of it and corrected it. Of course "mercy" is the KJV rendering here, but also in 4:1, although not in 6:4. RSV has "steadfast love" here, although oddly "kindness" in 4:1.
I suspect that the real reason is because 6:6 is quoted in the New Testament, in Matthew 9:13 and 12:7. But the quotation is taken from the LXX rendering of this verse which is itself apparently a mistranslation. That is, the Greek word eleos used to render Hebrew hesed, here and in most other places in LXX and by Matthew, does not normally have the same meaning as hesed. Rather, it means "mercy", and so in Matthew is correctly translated "mercy". The problem apparently came when the translators decided that the text in Hosea must be adjusted to fit Matthew's use of the verse, as they understood it. Unfortunately by doing so they managed to completely mess up their translation of Hosea, removing the clear markers of cohesion in this prophecy and destroying its sense.
This is one of a number of cases where an Old Testament passage is quoted in the New Testament in a rather different sense from what was intended by the OT author. If a translation of the OT is to be valid, it must be based on the intention of its own author, not on how it was understood by a NT author.
The NIV Old Testament is sadly marred in a number of places by misplaced attempts to harmonise with the New Testament. TNIV is a definite improvement in that a number of these poor renderings have been corrected. Unfortunately these corrections have not been made consistently in Hosea.