An apparatus and a method of operating a data processing apparatus, and simulators thereof, are disclosed. Data processing circuitry performs data processing operations in response to instructions, where some sets of instructions may be defined as a transaction which are to be performed atomically with respect to other operations performed by the data processing circuitry. When a synchronous exception occurs during a transaction the transaction is aborted and an exception counter is incremented. When the counter reaches a threshold value a transaction failure signal is generated, allowing, if appropriate a response to this number of exceptions causing transaction aborts to be carried out.