I've almost certainly missed a rule, but this makes me laugh so hard I wanted to share. It depends on two blinkered rulings, though.
Take a Cloistered Cleric 1. Your domains are Summoner (+2 CL to conjuration(summoning)) and either Trickery, Dragon, or Envy (Bluff as a class skill). You also have enough skill points for Know (Planes) 4, Bluff 4, and buying celestial and infernal straight up. Your other choice is regular cleric and some extra Int.
You are a human with two flaws and one trait. First level feats are Spell Focus (Conjuration), Augment Summoning, Extend Spell, and Sanctum Spell. Your trait is Spellgifted (+1 CL to one school, -1 CL to other schools. Pick Conjuration.)
Gain depravity (Heroes of Horror). At moderate depravity, take Eldritch Corruption as your bonus feat. At severe depravity, take Extra Slot.
Blinkered ruling one: Level 1 + 2 (Summoner Domain) + 1 (Spellgifted) gives you the 4 caster levels to qualify for Extra Slot.
Blinkered ruling two: Level 1 spell + Sanctum Spell + Eldritch Corruption is a 4th level spell so Extra Slot grants a 3rd level slot.
You can now cast summon monster straight up.
Has this been done? It's hard to know what to search on.