At 14:06 PDT, United Airlines Boeing 737 encountered turbulence at FL270 near Santa Barbara, injuring one flight attendant seriously. Another flight attendant and 11 passengers sustained minor injuries. The flight had just leveled at FL270 and was crossing the San Marcus VOR when the turbulence was encountered. The aircraft was 10 miles in trail of the preceding aircraft on that route when the event occurred. No significant weather was forecast on the high altitude prognostic charts.