1. A method of performing a program operation in a nonvolatile memory device, comprising: (a) applying a program pulse to selected memory cells; (b) detecting a number of fail bits among the selected memory cells, the fail bits comprising failed program bits and disturbed inhibit bits; and (c) determining a program completion status of the program operation based on the number of detected fail bits, wherein the disturbed inhibit bits include at least one disturbed inhibit bit among erase bits or at least one disturbed inhibit bit among passed program bits.