Planting DNA Evidence

I have been watching different shows concerning DNA evidence such as CSI and NCIS, the episodes I have seen lately deal with the planting of someone elses DNA at a crime scene to frame them.

My questions are How often do cases like this occur in real forensics? and if this does happen what systems are in place to protect the innocent in such a scenario from going to prison on false charges?

I am doing a research paper for school to attempt to pick out fact or fiction from forensic TV shows and real life forensics and any help would be greatly appreciated as I am new to forensics and hope to get involved in it in the future.