If you can split the task such that the current item is re-scoped to be tractable (say, “scope out <new challenge> and come up with a plan”) and you create new

If you can split the task such that the current item is re-scoped to be tractable (say, “scope out <new challenge> and come up with a plan”) and you create new tasks for the unexpected work / finishing initial goal, then you can capture the best of both; you did make forward progress, you ticked an item off, and you learned something useful. You just didn’t complete what you originally hoped you would.