The human body is equipped with a remarkably effective defense system. Once a dangerous substance has gotten into your body, it needs to push its way past the cell walls containing Defensin, a protein designed to keep bad things out. While the invader struggles to get in, it is liable to be eaten by Granulocytes and Macrophages, or marked for attack by a passing B cell. Even once infiltration is achieved, the body will seek out the compromised cells and destroy them with Killer T and NK Cells. Infections and illnesses come, but the body almost always wins in the end.
But such a powerful defensive weapon has a cost, as the HIV virus reveals. A powerful immune system can be turned against the body, destroying what it was built to protect. Some allergies exhibit a similar phenomenon, where the immune system gets over eager in destroying foreign particles. A lack of defenses leaves a system vulnerable to attack, but excessively capable defense systems are liable to be misused by outside forces.
The lesson doesn't just apply to the body. Google is caught in a war against 'black hat' search engine optimization. Do you ever see spam in the comments of an article? They often don't expect you to click on the link to purchase cheap knock-off purses, but they do expect Google to see the link, and return the target website higher in search results. Huge swathes of the internet are made up of dummy webpages, computer generated, intended only to look legitimate so the links to a real webpage are followed by Google and considered more authoritative.
When these tricks work, Google is left redirecting its users to scams and overpriced stores. So it works hard to identify these tricks and nullify them. If Google catches you gaming its system, it will remove you from its indexes, forever cutting you off from the biggest provider of pageviews around. The threat of banishment from its indexes is a very effective deterrent.
It also creates a powerful weapon to be abused. If you can't rank your website higher, causing your competitors to get kicked out of the index is the next best thing. Performing black hat search engine optimizations on another website can trigger Google's defenses against an innocent target, bringing advantages to the perpetrator. This is making Google's job harder.
And then there's law. Allegations of police officers planting drugs on suspects has a long history. It's difficult to catch someone in the act of a crime like drug use, so possession is used as a reasonable marker of intent and criminalized. But possession can be faked by an enemy, turning the sizable defensive weapon of the criminal justice system to twisted ends. Very recently, after 8 months of being abused and ostracized, a man was cleared of possession of child pornography after an employee of his bragged about planting the photos on his computer. In this case justice was eventually served, but its certainly the case that not every framer is foolish enough to leave behind a trail or brag about his crime.
The improper triggering of defensive mechanisms is, to some extent, unavoidable. The cost of letting an attack succeed in any setting can be too great to sustain. Thus a balance must be found that will inevitably include false positives, be it the creation of allergies or the imprisonment of the innocent. Still, it's a very important lesson to remember that evidence is easy enough to manufacture. When the penalties for a crime can ruin a life, especially based on the ultimately circumstantial evidence of possession of outlawed goods, some extra care must be taken. Things aren't always what they seem.