Drifting around in a variety of forms in the sea of ideas in our culture is this persistent kernel that there is a big difference between the contents and meaning of the Old Testament, and the contents and meaning of the New Testament. This idea may be expressed in the word Testament itself which suggests there were multiple different contracts God engaged in with human beings which contradict or overlap with one another. This idea may also be imported into the very identity of God as if we have a God of wrath and judgment on the one hand, and a God of love and peace on the other.
At a surface level I simply concede the point that there are indeed differences between parts of the Old Testament and parts of the New Testament. I believe the differences are usually exaggerated in conversation. The roots of all of Jesus' and Paul's preaching can be found all over the place in the Old Testament. Ideas of peace, and forgiveness, and love of neighbor are prevalent in the Torah and the Prophets. Furthermore, many of the real differences are, in my opinion, cosmetic - more a function of the gap in time between the writing of the documents, shifts in the culture, and the audience for whom the authors were writing than a deep theological divide. There are as many or more differences within the Old Testament itself, or within the New Testament as there are between the two testaments, but it is no skin off my nose to concede that one could easily find apparent conflicts between the Old Testament and the New if you wanted to.
This leads to a big problem though, because it puts us in the silly position of asking questions like "Why does God break God's own rules?" In the Torah God is depicted dictating the laws of the covenant to Moses who writes them down and delivers them to the people of Israel. In the New Testament, God is depicted as Jesus the Christ who comes in person and precedes to dismantle a number of the laws of the Old Testament. Paul and Peter follow suit in proclaiming God's covenant open to Gentiles, even those who follow none of the kosher food laws and who are not circumcised. There are a variety of possible (bad) solutions to this contradiction:
#1 - God changed God's mind. This is the solution of Open Theists and others who argue that God evolves over time and can change her opinion on things. There is Biblical support for the idea that God can change her mind- God does it several times in Genesis alone. To my mind this idea is lacking because it raises our understanding of God above God. It suggests that we understood God correctly then, and understand God correctly now and if the situation has changed the factor which is mutable must be God. As though it weren't more likely that it was we humans of limited perception that had misunderstood rather than that God had been unclear or inconsistent.
#2 - God's Laws Had An Expiration Date. This is very similar to solution #1, it is the idea of dominionists who believe that the history of God's providence is divided into eras and certain rules apply only in their respective era. I can agree with the idea that laws and ideas are contextual - ie: they matter to a certain time and culture in ways they may not to other cultures in other times, but I don't think this way of reading scripture really flies. Firstly, it chops the Bible up in ways that scripture doesn't seem to support. Jesus seemed to believe that his mission was the continuation and fulfillment of God's previous covenants with Israel, not a replacement for them. God's promises to Abraham and David didn't contain expiration dates. This ultimately makes God into an Indian Giver who promises things, but then takes them back after some hidden annulment clause triggers.
#3 - Only Some OT Rules Were From God. This solution basically says God is perfectly consistent, but only some parts of the OT were truly divinely inspired. Others were mistakes. Those mistakes got corrected in the NT and now we have it right. While I am agreed that the Bible isn't a "flat" text, meaning not every verse is equally important, I think this idea is ultimately unhelpful. How can we be sure we know exactly which rules were from God and which ones were not? The entire Torah is supposedly dictated by God directly to Moses. If they got that wrong, why should we believe that Matthew, Mark, Luke, John, Paul, Peter and the others got it so right? We can't start going at our Bible with scissors, that undermines the entire idea of inspiration.
All of these solutions are bad in my opinion because they are using the Bible incorrectly. The assumption behind each one of these approaches is that inspiration is in the text, as though it were a mineral to be mined out of the words on the page. All we have to do is use the right tools and we'll get the answer out of there. The truth (God) is contained in the black ink. That is why the solutions above either entail changing our ideas about God (#1) since the text itself seems contradictory we resolve the contradiction by making God flexible to our interpretation; or they involve adding something into the text (#2) like an entire mythological structure to history which makes sense of the contradictions; or they cut out the parts of the text we don't like (#3) until we can force it to make sense.
A better understanding of inspiration is that it is what occurs when the Holy Spirit intervenes between the believer and the text to reveal something true about God. The text is a dead letter. It is nothing but the human record of various people who had experiences of God through the Holy Spirit and tried to write them down. We return to them because of the witness of millions that they receive the inspiration of the Holy Spirit by faithfully engaging with them as scripture. Believers are what get inspired, not books. So here would be my solution to the apparent contradiction of God breaking God's own rules:
#4 - She Doesn't. The rules in Scripture are not God's rules and they never were. They are rules people of faith have written based on their best understanding of God's will. They are rules that people have sometimes followed and sometimes found that they led to a deeper relationship with God. But they are not "the Truth". Only God, revealed in Jesus is "the Truth" - and she is forever transcendent, forever undomesticated, forever surpassing our comprehension. All of our rules, and words, and images, and beliefs, and theologies, are sand mandalas that should be swept away every so often to remind us how fragile and impermanent our ideas about God are. To borrow a parable from Buddhism, we are standing on the shore of a vast lake and we want to get to the other side. Christianity is a raft that will take us to the other side of the lake. We should get into the raft and trust it to carry us across the lake, but when we get to the other side we're going to leave the raft behind, because the raft was never the goal. The raft was a vehicle to get us to the goal - the other side of the lake.