A circuit for a data processing apparatus and a method for detecting spurious signals is disclosed, the circuit comprising a data input operable to receive digital signal values, spurious signal detection logic operable to monitor a digital signal value within the circuit, and to determine at least one of: a safe time window during which it is expected that the digital signal values input into the circuit may cause data transitions in the monitored digital signal value and a transition time window in which it is expected a data transition will occur; and in response to detecting either a data transition in the monitored digital signal value outside of the at least one safe time window or no data transition in the transition window, the spurious signal detection logic is operable to output a detection signal.