Changes in skin's fatty underlayer where insulin is injected. Lipoatrophy is a depression in the skin; lipohypertrophy is a thickening or enlargement (raised area) of tissue.
Injecting any insulin at the same site repeatedly over time can cause a lipodystrophy: either lipoatrophy or lipohypertrophy. Either makes absorption unreliable, and can possibly lead to hypoglycemia[1] or hyperglycemia because insulin is not absorbed well from thickened tissue areas. But varying the injection site can cause variability in action profile, too. This BD Diabetes page[2] illustrates the most common areas humans with diabetes inject insulin and explains how absorption differs in various areas of the human body. This is true for ALL insulins.
Rotating injection sites can be helpful in avoiding/eliminating these. At times, the injection technique is the cause and needs changing.