  We report on the discovery of a new kind of thermal pulse in intermediate
mass AGB stars. Deep dredge-up during normal thermal pulses on the AGB leads to
the formation of a long, unburnt tail to the helium profile. Eventually the
tail ignites under partially degenerate conditions producing a strong shell
flash with very deep subsequent dredge-up. The carbon content of the intershell
convective region (X_C ~ 0.6) is substantially higher than in a normal thermal
pulse (X_C ~ 0.25) and about 4 times more carbon is dredged-up than in a normal
pulse.
