1. A data processing apparatus comprising: processing circuitry to perform data processing operations, the processing circuitry being operable in a secure domain and a less secure domain; a stack pointer register to store a stack pointer value; stack access circuitry to, in response to an event requiring a transition from the secure domain to the less secure domain, store a data item to a secure stack within a data store at a given location determined relative to the stack pointer value, the data item comprising a return address when said event is of a first event type, and comprising a first value when said event is of a second event type; and fault checking circuitry configured, on receipt of a first event type return from the less secure domain, to identify a first fault condition if the data item stored at the given location comprises the first value indicating that said event was of the second event type, wherein one of the first event type and the second event type comprises a function call, and the other of said first event type and the second event type comprises an exception.