Make the feds mad, and they will plant child porn on your computer. This is SOP. Very effective.
The problem with planting digital evidence is that an astute forensics expert could very easy do a bitwise extrapolation of data to find out if that file existed before the police broke down your door. And since the data on that disk must be made available to the defense lest they want a mistrial, they can’t obfuscate the truth.
I would not doubt it. I am told they also do this with illicit drugs.