It is extremely important to consult a veterinarian with expertise in lameness for pre-purchase evaluation of competitive riding (rather than breeding) horses. Pre-purchase evaluations should be conducted in the utmost objective fashion. If you are a buyer, it helps to bring in a veterinarian that is unfamiliar with the horse. Alternatively, ask for permission to see the medical records if the "regular veterinarian" is performing the pre-purchase. Some lamenesses are acceptable, but never be pressured into buying a lame horse.
For more information, please read the story Pre-Purchase Exams.