This post is, basically, USA specific. Your nation is probably not as bizzare as ours is. Feel free to compare/contrast :)
Good story on this topic done by This American Life, www.thislife.org in 2002, on 4/19, show #210. You can listen for free.
Three teens were convicted of a brutal rape/murder and served 15 years - the whole time they had blood type evidence on their side. One of the rapists had to be an O secreter, and none of them are. DNA evidence finally cleared them, but you can listen and hear how close they came to being imprisoned essentially forever, falsly - with clear cut evidence on their side that - at a minimum - there was someone else involved.
Oh ya, the fourth teen plea bargined: sent the rest to prison for life so that he would only get 2-3 years. It is this person's testimony (that basically the 4 of them did it) that was contradicted by the blood type evidence.
No one cared. In the heat of the moment, the police were under pressure to solve the crime and persuaded these boys to go along. Long after, for 15 years, they appealed, unsuccessfully on the O secreter evidence - and in the clear dispassionate light of day - they were turned down time and time again.
The second story on that episode is about how police got a 14 year old to confess to murdering his sister.
The system is so bad the state of Illinois suspended use of the death penalty.
Yet there is another problem which may be equally severe: disproportionate sentencing. So everyone wants to look tough on crime, and at the Federal level, since they don't really enforce laws like murder or rape, congress looked tough by passing mandatory minimum sentencing laws for drugs. There is something about interstate commerce in the constitution which in a strange way lead to this particular class of crimes being focused on.
Since the Federal government did not pay the states to fund this mandate, nor accept the prisoners for part or all of the mandatory sentences, the states essentially had to either build massive numbers of new prisons (thus looking tough on crime) or try to build some but not enough and effectively reduce the sentences for all other crimes...you know like rape and murder.
Obviously, looking tough on crime, but raising taxes would be unpopular so other state services may be cut, too. This is not my main point here.
When you hear about violent criminals going free after very brief jail times this is generally why.
In response to this nasty problem, the California voters implemented the "3 strikes" law, in which after 2 convictions for violent crimes, any third felony ensures life in prison.
As you can imagine this added to the drug sentences has had a huge impact on the prison population here. There is a consistent trend to higher University fees, fewer state services, and higher taxes.
The insanity of the 3rd strike being *any* felony became a topic of local humor. In public bathrooms where signs are posted saying "Felony to tamper with/destory/disable..." one often would find "three strikes and you're out baby!" scrawled across it.
Of course prosecutors have to *ask* for the three strikes law to be implemented, but as their careers are built on the severity of their victories you can imagine what happens. I'm sure no one is in prison indefinately for vandalizing a soap dispensor but....
The latest twist is that we are going to be asked to vote on another "Megan's Law" type protect the child bill. I'm not sure the actual content, but it has a rider which will change 3 strikes to ensure the 3rd strike is, actually, a violent crime.
Many times since 3 strikes the legislature has tried to change the 3rd strike to a violent felony only. They failed. Okay: this is a state that had, after the 2000 election, all statewide offices held by the liberal party (Democrats) and both chambers of the state legislature as well.
California. Strange place.