Scar tissue builds up as your body deposits collagen at the site of an injury. Normally, your body will reabsorb the collagen as the wound heals, but repeated injury to quickly at the same site can leave a build up of collagen. As others have noted, this can impede the absorption, but even worse, you may end up having a very uneven absorption. This variability is also a big problem, on day your bolus just doesn't work and the next you have a hypo. I am a firm believer in the ability of our bodies to heal. Even Richard157 who is a Joslin 50 year medalist talks about given parts of his body a time to rest and recover. These problems can be even worse with a pump. Marie's advice to really work hard to widely rotate injection sites is very wise indeed.