In Moldvay, possibly Holmes, and the retro-clone, Swords & Wizardry, Clerics simultaneously unlock 3rd and 4th level spells at the 6th experience level. They advance from 2/2 at level 5, to 2/2/1/1 at level 6.
I always put this down to a mistake, because it is gone in Mentzer. Why did Clerics advance in this way, and why is it repeated in the retro-clone S&W?
Why do Clerics then unlock 5th level spells the very next experience level (7th)?
Best Answer
D&D
Moldvay only covers to level 6. Cook wrote expert. However, Moldvay states:
No third level clerical spells are included in Moldvay.
Cook Expert shows your hop:
Classic D&D (little brown book) also shows this hop, however
It's likely a typo in the OD&D rules, and matches Cook's Expert, but note that both of those are different from Moldvay, where level 6 has no 3rd level spells.
By AD&D PH, it was fixed:
Likewise, in Mentzer:
I contacted a friend with Holmes basic; it covers ONLY levels 1-3; while it has 2nd level clerical spells, it has no provisions for characters to cast them.
S&W
Swords and Wizardry includes this jump because it was in the game it clones: classic D&D in it's brown or white cover era. S&W makes only a couple changes mechanically. None of them are in character capability, only in resolution mechanics, and the most notable being ascending AC, which is mathematically equivalent.
If you want to "fix" it
If you want a smoother progression, the following is a better fit with later lines on the chart for Classic, Moldvay, and Cook, and by extension, S&W:
I penciled this into a copy of Cook Expert some time around 1982...