[JustSayNoToNannies]

if it’s true a cop can on purpose or unintentionally cause a dog to give a false positive

It's true:

Handlers' Beliefs Influence Drug Sniffing Dogs' Performance-UC Davis Study-18 Dog Detection Teams, Over 200 False Positives (thanks to fattigermaster)

The original paper: http://link.springer.com/article/10.1007/s10071-010-0373-2/fulltext.html (amusing discussion of same at http://www.freerepublic.com/focus/f-backroom/2991974/posts?page=114#114 and following)

[1010RD]

One of the scariest things defense attorneys talk about is the “presumption of guilt” that persists among the public and jurors. If you’re black, you don’t have a chance.