I bought a replacement battery for my 2006 Macbook Pro and it started acting funny after about 6 months (the original lasted a while and had run its course). Went to the Apple Store, they did a diagnosis test on it, determined it was bad, and handed me a free replacement.

4 months later the replacement started having issues too. They tested it, found it was bad, and gave me another free replacement. That one's going on 8 months strong without issues.

I asked the tech why this was happening, if it was an issue with my laptop or something. He said that sometimes batteries go bad. It's the nature of the beast.