An apparatus and method are provided for controlling debugging of program instructions that include a transaction, where the transaction is executed on processing circuitry and comprises a number of program instructions that execute to generate updates to state data, and where those updates are only committed if the transaction completes without a conflict. In addition to the processing circuitry, the apparatus has control storage for storing at least one watchpoint identifier, and the processing circuitry is then arranged, when detecting a watchpoint match condition with reference to the at least one watchpoint identifier during execution of a program instruction within the transaction, to create a pending watchpoint debug event. The processing circuitry is then responsive to execution of the transaction finishing to initiate a watchpoint debug event for the pending watchpoint debug event. However, if instead the transaction is aborted before it finishes (due to a conflict arising), the processing circuitry is arranged to cancel the pending watchpoint debug event. Such an approach prevents a probe effect arising during execution of a transaction due to debugging activity.