When is the most appropriate time in a program's execution to reset a global error flag to false?

It is most appropriate to reset a global error flag to false immediately *before* initiating a new, critical operation or a sequence of operations that could potentially generate a new error. This ensures a clean slate for error detection specific to the upcoming task. It can also be reset *after* an error has been fully handled and resolved, signifying a return to a stable state, but the 'before operation' reset is crucial for accurate detection.