Am I missing something?

[Skevvix]

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Your spellthief levels stack with levels of other arcane spellcaster classes (that is, levels of any class that grants arcane spellcasting other than the spellthief) for the purpose of determining what level of spell you can steal. For example, a 4th-level spellthief/4th-level wizard could steal spells of up to 4th level, as if he were an 8th-level spellthief. Your spellthief and arcane spellcaster levels also stack when determining your caster level for all arcane spells. The character described above would have a caster level of 8th for both his spellthief spells and his wizard spells. In addition, you do not incur a chance of arcane spell failure for arcane spells cast or stolen from other classes, but only if you are wearing light armor. You incur the normal arcane spell failure chance when wearing medium or heavy armor or when using a shield.
Normal

A spellthief does not incur a chance of arcane spell failure when casting spellthief spells in light armor. He incurs the normal arcane spell failure chance for other arcane spells he casts, including those stolen from arcane casters (Complete Adventurer 15).

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Spellcasting: Knights of the Weave cast arcane spells like a sorcerer (you use your Charisma score to determine bonus spells per day, save DCs, and so on). Refer to Table 4—3 to determine your spells known and spells per day. You incur the normal arcane spell failure chance for wearing armor (but see Armored Caster, below). Your caster level for your knight spells is equal to your knight class level plus any other arcane caster levels you may have.

This class's spellcasting progression allows you to quickly approach the highest-level spells available to a single-class spellcaster of your character level, but you will always know fewer spells and be able to cast fewer spells per day than a single-class spellcaster.

So if I had say... Spellthief1/Wiz5/WarWeaver5/KotW1, wouldn't my CL be infinite?

Like...
Stf1    CL1
Wiz1  CL1/1
wiz2  CL1/2
Wiz3  CL1/3
wiz4  CL1/4
Wiz5  CL6/6(taking Master Spellthief at lvl 6)
WW1  CL6/6(no progression at WW1)
ww2  CL 7/7
ww3  CL8/8
ww4  CL9/9
ww5  CL10/10
KotW CL10/10/21(Stf bumps now to 31.  KotW now goes to 42-Stf to 52-KotW to 63 so on and so forth?)

[Bauglir]

Yes. I wrote a handbook in the past that I now don't agree with, but in this case the wording is reasonably clear. Master Spellthief is determined by arcane spellcaster levels, not by arcane caster level. It runs on class level. Let's assume a friendly DM, though, so your prestige class levels count whether they grant bonus CL or not (because the class is a arcane spellcaster). All your class levels, then, are eligible for Master Spellthief. You stack all your class levels to determine your CL in all classes, so you're at CL 12/12, which makes your Knight CL 12+12+1=25. The "loop" terminates here, because this is a stable result - we've just done Knight's calculation, and Master Spellthief doesn't care what your Knight CL is (just your Knight class level).

[Skevvix]

Thanks.  I was up for way to long, forcing myself onto graveyard shift, so I knew I was missing something.  Thanks again.