The rare instances where police plant drugs on suspects likely happens more often than it's caught, and planting DNA at a crime scene or asserting they got their sample from the scene is only marginally more trivial.

How is an investigator asserting that they found the trace DNA from hair or saliva they are presenting as evidence at the crime scene any different from any other assertion?

While these are exceptions, horrible abuses have already been committed by lying forensic specialists. In Canada, we had a forensic lab fabricate drug use evidence against mothers, who ended up losing their children. http://projects.thestar.com/motherisk/ , similarly, a pathologist was charged for fabricating evidence as well, https://en.wikipedia.org/wiki/Charles_Smith_(pathologist) .

These are low likelihood scenarios, but with an extreme impact that has consequences for the integrity of the system. These are in recent memory, and only examples who were caught.