Saul tried to take on the role of priest and make sacrifices to God before a pending battle and was cursed as a result
1 Samuel 13: 6 - 14
6 When the men of Israel saw that they were in trouble (for the people were hard pressed), the people hid themselves in caves and in holes and in rocks and in tombs and in cisterns,7 and some Hebrews crossed the fords of the Jordan to the land of Gad and Gilead. Saul was still at Gilgal, and all the people followed him trembling.8 He waited seven days, the time appointed by Samuel. But Samuel did not come to Gilgal, and the people were scattering from him.9 So Saul said, “Bring the burnt offering here to me, and the peace offerings.” And he offered the burnt offering.10 As soon as he had finished offering the burnt offering, behold, Samuel came. And Saul went out to meet him and greet him.11 Samuel said, “What have you done?” And Saul said, “When I saw that the people were scattering from me, and that you did not come within the days appointed, and that the Philistines had mustered at Michmash,12 I said, ‘Now the Philistines will come down against me at Gilgal, and I have not sought the favor of the Lord.’ So I forced myself, and offered the burnt offering.”13 And Samuel said to Saul, “You have done foolishly. You have not kept the command of the Lord your God, with which he commanded you. For then the Lord would have established your kingdom over Israel forever.14 But now your kingdom shall not continue. The Lord has sought out a man after his own heart, and the Lord has commanded him to be prince[b] over his people, because you have not kept what the Lord commanded you.”

David eats the bread of the presence
1 Samuel 21: 1 - 6
Then David came to Nob, to Ahimelech the priest. And Ahimelech came to meet David, trembling, and said to him, “Why are you alone, and no one with you?”2 And David said to Ahimelech the priest, “The king has charged me with a matter and said to me, ‘Let no one know anything of the matter about which I send you, and with which I have charged you.’ I have made an appointment with the young men for such and such a place.3 Now then, what do you have on hand? Give me five loaves of bread, or whatever is here.”4 And the priest answered David, “I have no common bread on hand, but there is holy bread—if the young men have kept themselves from women.”5 And David answered the priest, “Truly women have been kept from us as always when I go on an expedition. The vessels of the young men are holy even when it is an ordinary journey. How much more today will their vessels be holy?”6 So the priest gave him the holy bread, for there was no bread there but the bread of the Presence, which is removed from before the Lord, to be replaced by hot bread on the day it is taken away.

David puts on the ephod to inquire of God as to how they proceed and God honors him
1 Samuel 30: 3 - 9
3 And when David and his men came to the city, they found it burned with fire, and their wives and sons and daughters taken captive.4 Then David and the people who were with him raised their voices and wept until they had no more strength to weep.5 David's two wives also had been taken captive, Ahinoam of Jezreel and Abigail the widow of Nabal of Carmel.6 And David was greatly distressed, for the people spoke of stoning him, because all the people were bitter in soul,[b] each for his sons and daughters. But David strengthened himself in the Lord his God.
7 And David said to Abiathar the priest, the son of Ahimelech, “Bring me the ephod.” So Abiathar brought the ephod to David.8 And David inquired of the Lord, “Shall I pursue after this band? Shall I overtake them?” He answered him, “Pursue, for you shall surely overtake and shall surely rescue.”9 So David set out, and the six hundred men who were with him, and they came to the brook Besor, where those who were left behind stayed.

Both were in the position of kings, but only was fit the role in its entirety.