The case of Timothy Young made national headlines in 2012 when New Mexico police anally probed him in search of drugs (no contraband was found). His ordeal was the result of a false positive alert by a drug-sniffing police dog. Incredibly, the same dog was involved in a case involving another New Mexico resident that resulted in forced rectal exams that uncovered no drugs. That case ended with authorities paying a $1.6 million settlement (Young's case is still pending).
Although presented as impartial and infallible, it turns out that such dogs are not only often poorly trained, they are frequently wrong.