Getting the horse used to novel objects
I know I often catch myself thinking 'this horse has done X,Y or Z in the past so he will find this lesson hard'. On the other hand, I can also think that the same horse found a particular lesson easy and assume that the new movement would also be easy. Both of these assumptions will, most probably, lead me to teach the lesson differently. In the first instance I might not teach the horse as much - take him a little slower or spread the lesson over some time. The second instance can be more detrimental to the horse in many ways because I am more likely to skip over steps or make assumptions about what the horse knows. To avoid holes in your training try to take each individual lesson for each horse through the same series of steps regardless of the horse's history. In the long run you will end up with a horse that is solid and confident.