The double jeopady clause of the Fifth Amendement prevents a person from being convicted and punished twice for the same offense. It also prevents re-trial after a defendant is found not guilty. The protection extends not only to the charged offense, but also any lesser included offenses.
A recent Dallas County case provided a lesson in double jeopardy law. The defendant, Sharone Sylvester Brown was arrested for assaulting his girlfriend. He went to jail, and a couple of weeks later pled guilty to misdemeanor assault and was sentenced to 60 days. Several hours after he was arrested his girlfriend went to the hospital. She had cancer, and was had numerous medical issues as a result. She eventually passed away, and the initial determination was that it was due to unenexplained circumstances. However, that was later changed to homicide. The doctors concluded that while she died from cancer. the assault contributed to her death.
When prosecutors found out the girlfriend had died they arrested Brown for murder. Later they discovered they had to dismiss the case because he had already plead guilty to the assault. At the time of the plea the district attorney's office was not aware that the victim had passed away, or had even been taken to a hospital.
This is one where a quick plea benefits the defendant. He was only jail a couple of weeks when he plead guilty and was sentenced. Had he waited, he would be looking a murder charge instead of what he ended up with.