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 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 fault condition if the data item stored at the given location comprises the first value, 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.