In reference to mixing theurge classes with fast-progression classes like the Apostle of Peace, Blighter, or Ur-Preist this answer mentions that "Any time you get a spell level before a Cleric or Wizard would, you’re causing trouble". This has lead me to ask – is there any way to do this with consistency (i.e. not just at one particular level)?
Excluding obvious cheese like Pun-pun, I'm not aware of any tricks that would allow such a progression. Even among the classes that I've listed, I only know of one-off cases like the Apostle of Peace getting 9th level spells one level earlier than the Cleric, but that doesn't last long enough to really count. Do the exploits that the linked question alluded to have something to do with exploiting early entry exploits?
Note: To clarify the level of cheese that I'd be happy with – the best possible answer will be one that is only considered cheesy due to its end result (the faster than normal spell progression) rather than its means of getting there (e.g. using the sort of exploits that could give you Pun-pun).