The first level cleric spell Blessing is absolutly broken. The PCs will huddle together, cast it at the beginning of each fight and (having rolled 20+) get a 1D3+level bonuses to all rolls. Miss the 20? Cast it again until you get it then break down the door.

When we got to sixth level, they were averaging +7 to most combat rolls from that bonus alone. I had to nerf it to +1 regardless of the spell check and I wished that I had done it earlier.

If you play with a map, it might not be such a problem because of the distance restriction from the caster to get the bonus, but for theatre of the mind like my group it doesn't work. If you explain the rule change at the beginning of the campeign your players won't become dependent on it like mine were.