I had a wide range of riders and horses when it comes to their skill levels. Also I worked with Western riders. This made my work very diverse and kept me on my toes :) However, there are timeless basics and fundamentals that all riders need to be reminded of. Though, before I list the common challenges riders worked on during the clinic I want to praise their open mindedness, willingness to go beyond familiar and comfortable, willingness to feel weird and to listen to their horses for the truth.

Locking knees, using stirrups for support - This issue is quite common everywhere. As soon as rider stiffens up the knees looking for support from stirrups she/he is bound to unplug the seat from the saddle. Partially it is a reflex of our body which is seeking a base of support. However, stirrups are not stable and the body locks the knees in attempt to stabilize itself.
Losing contact with a bit - This is for English riders. It is very important to learn not to loose contact with a bit. I do not say reins or horse's mouth. I mean the bit. Riders are very often "ring" the reins trying to be soft or make their horses soft. This makes the bit rattle in horse's mouth. This is quite unpleasant for the horse. Even if you need to move the bit always do it slowly without losing contact with it.
Trying to "help" the horse to turn by turning upper body inward and moving hands like if riding a bicycle - I sometimes think this happening to riders because most of us learn to ride bicycle before we learn to ride a horse. Riders move their hands in turns exactly like if they were holding the handles of a bicycle. This makes inside of the body collapse to give way to the arm that is moving backwards and encourages the shoulders to turn inward too much. Horses start mimicking the rider's posture and collapse themselves.
Be ready to hold a heavy load - This one is not easy to explain. First of all many people do not lift/hold a heavy load properly - using legs with bent knees, back straight, the load is close to the body, upper arm close to vertical position, holding the load with the whole frame not just hands, shoulders, or lower back. When riding there a chance for a horse to become heavy on the reins, pull, etc. The reasons for it are too many to list. However, riders must sit in such a way that this sudden increase of weight in their hands will not affect their posture and weight is distributed over the whole body including legs rather than trying to hold it with stiff outstretched hands.
Not paying attention to the path and the speed - Riders sometimes are not aware of how fast/slow their horses are moving and where exactly their feet are going. Starting from beginner riders who let their horses wanter around arena to advanced riders who are not realizing their horses slowed down through the turn, cut a corner, or finished a movement earlier or later than planned. Working on going exactly where rider wants with exact speed is a powerful mental exercise for a horse. It also teaches the rider to stay focused and precise.
Pulling on the reins during unrequested back up - Unrequested back ups often happen when a rider has an argument with her/his horse. At this moment if the horse starts moving backwards all on his own it is scary and unbalancing. The rider seeks support and starts subconsciously pulling on the reins. This is very dangerous because the horse will either go backwards faster or rear. It happened last year at Olympic games in London. It was jumping phase during modern pentathlon. A horse started acting up and went backwards, the rider pulled on the reins, the horse reared and the rider fell down. If you didn't ask your horse to back up and your horse went backwards immediately push your hands forward and kick with your legs until your horse makes a forward step.
Focusing too much on inside aids in shoulder-in - In shoulder-in riders forget the fact that horse though bent to the inside must travel on a straight line. In order to do so the horse is pushing his body from inside to the outside. If riders focus too much on bending they end up leaning slightly in and loading inside seatbone too much. The result of this a horse that drifts away from the wall following rider's weight. Sitting absolutely vertical and receiving the horse's energy with outside aids is of great importance for successful shoulder-in.
Not controlling horse's inside shoulder in travers - Riders end up focusing too much on pushing haunches to the inside forgetting the fact that horses are rather horizontal creatures and have two ends that require control: front and back. One of the main reasons riders lose bend or path in travers is because they lose inside shoulder by letting it escape to the inside. The horse starts leg-yielding rather then traversing.
Losing activity in canter - Riding good canter is a great feeling. Not all horses are blessed with super light, balanced and naturally engaged canter. Most of them must work to achieve it. In this situation it is very important not to lose pure three beat and active jump every canter stride. Riding lots of forward transitions, especially, on big circles. Teaching a horse to actively push off the ground when a rider closes both legs. Paying special attention to quality of the canter, especially, when working on collection.