I was out riding with one of my usual riding buddies and his gf. He's very experienced, rides a ZX10R, and he and I ride about the same pace.
She is a very inexperienced rider, she's put maybe 1000-1500 miles on her Ninja 250 in the last two years she's owned it.
We were riding with him in front, her behind him, and me at the rear in a staggered formation. We were riding along in pretty heavy traffic and paying attention in front of us as well as to the car trying to merge into us from the right. The cars in front of us braked suddenly and all 3 of us hit the brakes pretty hard. Here's where the issue comes in.
You need to use the front brake!
I've noticed with a lot of Ninja 250 riders that they use their rear brake the most. I understand why, seeing as both are equally effective on that bike, not to mention that the front causes the weak forks to dive. That said, it's a bad, horrible, terrible habit. She locked up her rear brake and the rear end kicked out to the right. The bike went down and she tumbled off into the lane to the left. Her helmet is scraped all to hell, but her TourMaster jacket held up extremely well. She is a little torn up, some fatty tissue missing from her knee, a decent bruise on her hip, and a scraped up hand (wear sturdy gear people, those thin gloves are next to useless). She'll be ok, but the bike is probably totaled. We had probably 10 people stop and make sure she was alright, and blocking traffic for us, so that was absolutely terrific.
Another side note is that I've always wondered if staggered formation was really the way you should be riding, and after seeing what I saw today, I can tell you that it's exactly how you should ride. The bike in front was to the right, her bike to the left, and my bike to the left. When she went down, the bike went forward and slightly to the right, missing my buddy/her bf's front wheel by about 3 feet. I was able to stop before I hit any part of her bike or her solely because I was off to the side.