I've been waiting a while for this one. I've always said I'd rather be around a million halfway decently bred pit bulls (including game bred ones) than one shoddily bred Border Collie.
Even well bred Border Collies have much potential for disaster. The Border Collie Boards, the forum for the breed club, (you can google it if you are curious) is filled with cries for help from people with fearful, snappish, aggressive, reactive dogs from every source.
A Border Collie has lightning quick reflexes, makes decisions quickly, puts two and two together in ways that boggle the mind, and it's a breed that is meant to act independently to "fix" perceived problems.
People, on the other hand, have been told for many years that it's wrong to say no to - well, to anything, much less the dog. The Border Collie is a breed that needs rules, or it will make up its own. Some of those may include, "No touching me. No coming within twenty feet. Men wearing hats are Beelzebub." The Border Collie is the master of the random association, and without structure the randomness can spread wildly.
Enforcement of the rules depends on the personality of the dog. Some dogs just shut down. Some try to run off. Some go on strike or play dumb. And some bite. Many bite. And unlike breeds not designed to work closely with dangerous livestock, when a Border Collie aims a bite, it will probably land that bite. See above about the lightning quick reflexes, etc.
I've worked with aggressive BCs for years, and their owners. I've sat there and pointed out the signs of stress and aggression in the dogs to owners and they still don't see them, because they are so subtle. I can smell a bite coming a mile a way, but it took working with dozens of dogs to get to that point.
Border Collies are smart and fast, but fortunately they are pretty much all big chickens, too. Actually, the fact is that 90% of the aggression I've worked with came from the dog not knowing his or her place in life. Not in a dominance way, but not being part of a team. Border Collies don't all need 10 hours of agility, flyball, or frisbee, or 15 miles of walks. In fact most of them don't. I've seen a dog that was straight out of top working lines and trained to a high level himself, that was happy as a clam as the companion of an 80 year old couple whose idea of exercise was to ride out to the mailbox in the golf cart. I knew another dog who lived in an apartment and fetched beer from the fridge for his bachelor human housemate.
The secret was that there was a clear structure and rhythm to these dogs' lives, and they could trust their human companions to make good choices for them. Those people were the Coolest People on Earth, and they didn't have to worry about things that move, or odd looking things in the environment, or scary noises. It takes some time, dedication, and a bit of knowhow to get a dog to that point, but once it happens, the payoff is hugely worth it.
Border Collies are odd creatures. They are a balance of such extremes. On the one hand, the dog I see at herding clinics with his 80 year old human friends, levelheaded no matter what's going on, off leash the whole time, yet ready in an instant to work when his trainer (the clinician) calls, a picture of barely contained savagery as he flings himself to and fro at Jack's whistles to work the sheep. Then he returns again to his owners, content to lay there and gaze with adoration at them - somehow recognizing that they were the ones who gave him that gift.
On the other hand, the spoiled dogs I get here - out of control, won't look a human in the eye, their eyes roving constantly for anything that will offer some sense to the world, flinging themselves to the end of the leash, growling or snapping when I exchange the collar for one with my information. The owners both nervous at the thought of giving up their Snookums, yet terrified of Snookums (and Snookums definitely knows it!).
The first week is Quiet Time. This dog has returned to the whelping box, and I am its mother and only littermate. After that he will learn to respect first the leash, as it is an easy and concrete limit to set, and then depending on his personality and what he offers me, he'll learn to be a team player in some way that matches what is easiest for him.
Many times, by the third week, the second dog I describe more closely resembles the first dog. Border Collies don't really have an in between where they are jolly and clueless. That's the weird thing about them. They are like the Girl with the Curl in the Middle of Her Forehead. When they are good, they are very, very good, but when they are bad they are awful. Working trainers and breeders may try to blame Barbie Collie folks, Barbie Collie folks blame the working lines for breeding bad temperaments, and Sport Collie breeders blame both, but really it stems from too many people having these dogs that just don't understand a dog that needs to be needed.