One of the things I really enjoy appearing on my Facebook news feed is "instant karma" videos. The guy who kicked a cat, only to have a flowerpot land on his head. The dude who tried to knock his friend into the lake, only to fall in himself instead. Perhaps the best I've ever seen is the biker who tried to steal a woman's handbag. She threw it away, he got off the bike to chase it, so she stole his bike and rode away. These videos are hilarious. The problem is, they are not instant karma.
As I understand it (though I could be wrong), Karma made its way into our culture through the show My Name Is Earl, in which the titular character, a lowlife criminal, discovers the concept while in hospital. He understood it as the universe taking note of your behaviour and repaying you for it. If you do good things, good things happen, whereas if you do bad things, bad things happen. So, Earl makes a list of all the wrongs he's ever done, and the show revolves around him trying to make up for them one by one, hoping his life will then get better.
In reality, however, Karma is not about this life, but the next. Religions that teach reincarnation believe that your actions in this life dictate how you will live in the next. If you live a good life, you will be reincarnated into a good life. If you live a bad life, you will come back as something bad.
Imagine if you were brought into a courtroom, and it was completely empty. There is no judge, no jury, not even a bailiff to make sure nothing goes wrong. This is the major problem with Karma. It's ambiguous. By contrast, we as Christians know that there is a judge. But He doesn't decide what to do with you when you go back to Earth. Rather, He will judge the life you lived, and either punish you eternally according to your sins, or reward you eternally for all your good works.
Therefore, both understandings of Karma are wrong. In this life, wisdom reigns supreme. Living wisely does not guarantee a good life, neither does foolishness guarantee a bad life. However, the world does run on a system of predictable cause and effect. There is no "Karma" to determine what will happen to you if you do good, and in fact, it is more likely that choosing the good path will make you vulnerable to evil.
In the next life, how you lived will not determine how you return to this realm, but the quality of your eternity in the next. For those in Christ, our sins will be forgotten, and our good deeds will be measured. For those outside of Christ, the full measure of their sin will be returned to them in Hell. Christians need not fear having "bad Karma", but we should always seek to bring glory to God.