A variant on the Sha'ir stuff:
Take the Spontaneous Divination ACF as a wizard, and now you both can prepare spells from a spellbook and cast (certain) spells spontaneously. Thus, you qualify for Ultimate Magus. (This part I think is fairly unambiguous. The cheesy part is what comes next.)
By a really permissive reading of the rules, Ultimate Magus would advance your Wizard spellcasting twice, for the same reasons as with the Sha'ir. But this way you get Tier 1 spellcasting double advances, not the Sha'ir's inferior spellcasting, and you get the other perks of Ultimate Magus.
Of course, neither this nor the Sha'ir trick actually works, I think. And even if it did, it would be to unbearably cheesy for words.
Despite the cheese, it's a fun thought experiment though. And it almost works, if you look at it sideways and squint. What it really boils down to is the lack of a clear definition of what is meant by an arcane/spontaneous arcane/divine spellcasting class at all. (Which, strangely enough, is a really big oversight to leave undefined.)