It is narrated that the Prophet ﷺ said:

"O Allah, I seek refuge in Thee from the evil of what I did and from the evil of what I did not."

This prayer acts as a total safety net for your actions. It covers the evil of what was done (past mistakes and their consequences) and the evil of what was not done (missed opportunities or future harm). From a practical standpoint, it addresses both the errors you have already committed and the potential risks of things you haven't done yet. By covering both sides of the timeline, you create a complete "shield" for your life.