I believe that framing people for possession of child pornography is a widespread practice, and that this accounts for almost all convictions on that charge. I base this on the evidence that is typically used in such cases, all of which comes from computers which may have been compromised; and in fact, trials usually mention evidence that the computers in question were compromised (although it's possible for an attacker to remove all evidence of that fact), and that hasn't been a successful defense. If a person were to actually want child pornography, there are simple technical measures which could create a nearly iron-clad guarantee against being caught; and conversely, similar measures with a similar guarantee protect people from being caught planting evidence. Finally, the societal irrationality surrounding child pornography means that successfully getting someone accused of having it will not only get them jailed, but thoroughly destroy their reputation and shame them as well.