What are the most common rider errors that make counter-canter difficult for the horse?

The most damaging rider error in counter-canter is over-bending the horse's neck toward the direction of travel rather than keeping the horse's flexion aligned with the lead. If you're cantering right lead and tracking left, the horse's slight poll flexion should remain to the right — toward the leading leg — not pulled left to match the direction of movement. Pulling the nose toward the direction of travel collapses the horse's outside ribcage, destroys its balance, and makes the exercise nearly impossible. A close second error is collapsing the inside hip: many riders unconsciously shift their weight into the inside seat bone of the direction they're tracking rather than sitting with weight toward the leading leg. This confuses the horse's balance cues and often causes breaks to trot. Third is over-driving with both legs equally — counter-canter requires the same asymmetrical leg position as any canter, with the outside leg (relative to the lead) held back to maintain the gait. Riding with both legs at the girth often causes the horse to swap leads or lose impulsion. Finally, stiff or gripping hips are a common problem — counter-canter demands a supple, following seat. Riders who brace against the canter motion disrupt the horse's back and make the gymnastic work punishing rather than strengthening.