1. An apparatus comprising: processing circuitry to execute program instructions including a transaction comprising a number of program instructions that execute to generate updates to state data and to commit said updates if said transaction completes without a conflict; and control storage to store at least one watchpoint identifier; the processing circuitry being arranged, when detecting a watchpoint match condition with reference to the at least one watchpoint identifier during execution of a program instruction within said transaction, to create a pending watchpoint debug event; the processing circuitry being responsive to execution of the transaction finishing to initiate a watchpoint debug event for the pending watchpoint debug event; and the processing circuitry being responsive to a conflict arising during execution of the transaction to cancel the pending watchpoint debug event.