While a self-cleaning oven can save a lot of time and hassle in the kitchen, extremely dirty ovens require some pre-cleaning beforehand. Running the self-cleaning cycle with an extreme amount of grime can require multiple cycles and possibly even damage surfaces inside the oven.
The self-cleaning cycle uses extreme heat to incinerate food residue. Some types of spills become problematic at high temperature. Sugary residue may actually burst into flames, marring the interior of the oven's surface. Heavily caked on messes can block airflow, causing elements to overheat and possibly triggering an electrical failure. The more material there is to remove, the longer the oven has to work to burn it away.