Very recently a Cessna 421 got refueled at a midwest airport. The pilot told the line service to add 80 gallons of fuel, 40 on each side and then he left for the night. A long trip was planned for the next day. After take off the pilots noted a loss of power in both engines. Full RPMs but no oomph. The result, a Mayday call and a 180 turn. Unfortunately they couldn't make the airport and crashed in a field. They all survived, although the plane was mangled. The cause was easy to determine. The addition of Jet fuel instead of Av Gas.
( See: NTSB Jan'09 Case CEN09LA145). The two fuels just don't belong in the same tank! How could this have been avoided? Well for one: carefully communicate with the refueler or the desk clerk. Better than that: Stay there and watch the process of refueling. The extra time spent may save your life.