Fuel not being sumped regularly, fuel not received/used/turned regularly, filters not being changed, tanks not being inspected or cleaned regularly.
Yeah, a lot of self serves are used regularly and maintained well. The busier the airport the more likely the chances for a well cared for self-serve are. Personally I try not to go to BFE for fuel and when possible I prefer full-service that has a flag attached (Phillips, BP, AvFuel etc) because of the quality control that comes with it. It's not worth saving a couple cents a gallon to skimp on quality of fuel. And that's coming from a twin driver who has 163 gallon tanks to fill.