Before you start thinking that rider A is getting what he deserves, remember that in a tight pack a rider rarely falls without taking someone else out on the way down. Rider A can and should ride closely to rider B, but he should either overlap his wheel by more than half a wheel diameter (at the axel bolt) or not at all.
More: Basic Skills for Group Riding
"But wait," you say. "Isn't riding close what pack riding is all about?"
Yes and no. Pack riding is all about riding efficiently as a unit. In a sense, you are working together but recognizing each others' space and presence in the peloton. Each rider must be keenly aware of his or her surroundings. Sudden movements side to side or a hard application of your brakes will result in a harsh verbal response from your neighbors. Sometimes it's unavoidable to have physical contact with the riders around you. A shoulder bump here, arms brushing another rider there—these are common occurences in the peloton and should be expected, not feared.
More: 10 Rules to Group Ride Like a Pro
TIP #2: Use your peripheral vision at all times. Don't focus solely on the rider in front of you. Notice what the rider two lengths ahead is doing. Keep the rider on your left and right in your periphery. Are they dodging debris or a pothole? And when you are monitoring the rider in front of you, use her hips or back as a distance guage, not her rear wheel.
Keeping you head up will help you maintain balance and allow your peripheral vision to pick early warning signals from erratic riders around you.
More: 5 Common Bike-Handling Mistakes
TIP #3: Don't hesitate to use verbal cues and even a physical touch to let others know where you are or where you are moving inside the pack. A simple, "Here!" is often all that's needed to keep a rider in front from moving into your path.
If verbal warnings aren't effective or appropriate, reach out and touch the rider who is infringing on your path. A touch on the hip is a warning sign to the rider in front that he may cause a crash if he continues into your path. Note that pushing or hitting a rider is never a good idea. These actions can be grounds for disqualification if observed by a race official.
TIP #4: When you are up front, maintain a steady pace. Unless you are preparing for a final sprint or breakaway, your motion inside the pack should be gradual and predictable. Nobody wants an unsteady or "squirrily" rider in their peloton.
More: How to Overcome Your Fear of Crashing
Unless in a sprint, avoid accelerating too hard when standing up over a hill or after a turn. Your quick movements make it harder for riders behind you to ride close and trust your handling skills.
More: Learn to Ride Faster and Safer With Bike-Handling Drills
TIP #5: Smooth out the "slinky effect." If you are located on the back end of the pack, you will invariably be involved in numerous accelerations and decelerations. When trying to stay on, riders often accelerate and brake hard to maitain a tight formation. This can be counterproductive and dangerous. If you have 10 or more riders in front of you, the slipstream from their motion will be much larger than that of a smaller group. This means you can maintain a slightly larger gap between riders and still benefit from the reduced aerodymanic drag.
More: Drafting Principles: Improve Your Riding Economy
Conserve energy by anticipating accelerations and subsequent braking. If the riders in front of you jump to close a small gap, let your momentum and larger slipstream bring you back to the group when they brake again because the've accerated too hard.
Lastly, try to maintain calm. If you allow your nerves to gain control of your riding, others will know. They will sense your nervousness and become nervous themselves. A nervous pack is a dangerous pack. Try to relax and enjoy the ebb and flow of the communal peloton. At the same time, stay alert and keep watch of other riders.
With practice and by observing these tips, you will become a better pack rider who is ready and rested for the next points sprint or the sprint to the finish line.
More: 10 Secrets for Riding in a Peloton
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