A system for use in a well, comprising: a well tubing disposed in a borehole ; and an extrudable ball seat secured along the well tubing , the extrudable ball seat comprising: a larger diameter section ; a smaller diameter section ; and a conical section extending between the larger diameter section and the smaller diameter section , the conical section having an internal ball seat surface for receiving a plurality of different balls in sealing engagement, the smaller diameter section being formed of a material which expands to extrude a first ball of a first diameter under pressure, the smaller diameter section further comprising at least one notch located to initiate crack propagation when a second ball of a second diameter , larger than the first diameter and of at least a pre-set value, is extruded through the smaller diameter section when placed under pressure, wherein the at least one notch is provided in the smaller diameter section and not in the conical section and wherein the smaller diameter section and the conical section are constructed to enable extrusion of the first ball without initiating crack propagation. a well tubing disposed in a borehole ; and an extrudable ball seat secured along the well tubing , the extrudable ball seat comprising: a larger diameter section ; a smaller diameter section ; and a conical section extending between the larger diameter section and the smaller diameter section , the conical section having an internal ball seat surface for receiving a plurality of different balls in sealing engagement, the smaller diameter section being formed of a material which expands to extrude a first ball of a first diameter under pressure, the smaller diameter section further comprising at least one notch located to initiate crack propagation when a second ball of a second diameter , larger than the first diameter and of at least a pre-set value, is extruded through the smaller diameter section when placed under pressure, wherein the at least one notch is provided in the smaller diameter section and not in the conical section and wherein the smaller diameter section and the conical section are constructed to enable extrusion of the first ball without initiating crack propagation. a larger diameter section ; a smaller diameter section ; and a conical section extending between the larger diameter section and the smaller diameter section , the conical section having an internal ball seat surface for receiving a plurality of different balls in sealing engagement, the smaller diameter section being formed of a material which expands to extrude a first ball of a first diameter under pressure, the smaller diameter section further comprising at least one notch located to initiate crack propagation when a second ball of a second diameter , larger than the first diameter and of at least a pre-set value, is extruded through the smaller diameter section when placed under pressure, wherein the at least one notch is provided in the smaller diameter section and not in the conical section and wherein the smaller diameter section and the conical section are constructed to enable extrusion of the first ball without initiating crack propagation. <EOS>