Apple released firmware updates for MacBook Pro (mid 2012) and MacBook Air (mid 2012 and mid 2013) to address an issue with battery performance.
MacBook Air
Apple's patch notes.
MacBook Pro SMC Firmware Update 1.8
This update is recommended for MacBook Pro (mid 2012) models.
This update addresses rare issues where the battery unexpectedly stops functioning or the system hangs when the battery charge reaches less than one percent.
And
This update is recommended for MacBook Air (mid 2012 and mid 2013) models.
This update addresses rare issues where the battery unexpectedly stops functioning, the system does not recognize the battery, or the system hangs when the battery charge reaches less than one percent.
You can find download links at the URLs above. Affected models should also have the updates in the Mac App Store under Updates in Mountain Lion, or under Software Update in Lion.
Note that the firmware download for MacBook Pro is huge, at 703MB. The MacBook Air firmware update is a much more svelte 1.09MB.