A method for debugging program code is performed at an apparatus having one or more processors and memory for storing programs to be executed by the one or more processors. The method includes the following steps: for each program code segment from a set of program code segments, assigning a unique value to a detection variable of a state machine when the respective program code segment operates the state machine. The unique value is based on a value of an identification variable associated with the respective program code segment. The method also includes determining a current value of the detection variable corresponding to an abnormal program exit, and determining a suspicious program code segment based on a comparison of the current value of the detection variable with the unique values of the respective identification variables. The method further includes outputting debug information based on the suspicious program code segment.