The first and third benefits of the feat Master Spellthief from page 79 of Complete Scoundrel are pretty straightforward (although the first's a little sad in that it only affects the level of spell one can steal not the entire supernatural ability steal spells). But--whoah--that second benefit.... Some readings say there's the possibility of some ridiculous caster level shenanigans.
Your arcane studies allow you to mingle arcane magic of different flavors for great effect.
Prerequisite: Ability to cast 2nd-level arcane spells, steal spell.
Benefit: 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).
The prestige classes that seem to gain the most benefit from the feat Master Spellthief include...
The prestige class Green Star adept (CAr 41-4) at level 1 gains the extraordinary ability improved caster level, which says, "A Green Star adept adds his class level to his caster level in another arcane spellcasting class to determine his effective spellcaster level. If the character had more than one arcane spellcasting class before becoming a Green Star adept, the player must decide to which class to add each adept level for this purpose." At level 2 the green star adept gains the class feature spells per day/spells known, which says, "At every even-numbered level, a Green Star adept gains new spells per day (and spells known, if applicable) as if he had also gained a level in an arcane spellcasting class to which he belonged before adding the prestige class level."
Alex is a Spellthief 2 / Wizard 16 / Green Star Adept 2 who possesses the feat Master Spellthief. What's Alex's caster level for his wizard spells?
The prestige class knight of the weave (CV 111-7) at level 1 gains the class feature spellcasting, which says, "Your caster level for your knight [of the weave] spells is equal to your knight [of the weave] class level plus any other arcane caster levels you may have."
Bob is a Spellthief 4 / Wizard 15 / Knight of the Weave 1 who possesses the feat Master Spellthief. What's Bob's caster level for his A) spellthief spells, B) wizard spells, and C) knight of the weave spells?
The prestige class sublime chord (CAr 60-2) at level 1 gains the class feature spells per day which says, "A sublime chord’s caster level for both her sublime chord spells and the spells she gains from other arcane spellcasting classes is determined by adding her sublime chord level to her level in another arcane spellcasting class. If she had more than one arcane spellcasting class before becoming a sublime chord, she must choose to which class to add her sublime chord levels for the purpose of determining her sublime chord spellcaster level."
Chris is a Bard 1 / Spellthief 1 / Wizard 8 / Sublime Chord 10 who possesses the feat Master Spellthief. She applies the sublime chord class feature spells per day to her wizard casting. What's Chris's caster level for A) wizard spells and B) sublime chord spells?
The prestige class ultimate magus (CM 77-81) at level 1 grants the class feature spellcasting, which says, "At each level except 1st, 4th, and 7th, you gain new spells per day and an increase in caster level (and spells known, if applicable) as if you had also gained a level in both a prepared arcane casting class and a spontaneous arcane casting class to which you belonged before adding the prestige class level. [...] At 1st, 4th, and 7th level, you gain new spells per day and an increase in caster level (and spells known, if applicable) as if you had also gained a level in your arcane spellcasting class with the lowest caster level." Further, the class grants the extraordinary ability arcane spell power, which says, "At 1st level, your caster level for all arcane spells increases by 1. It increases again at 4th level, 7th level, and 10th level (to a maximum of +4)."
Dylan is a Sorcerer 1 / Spellthief 4 / Wizard 3 / Ultimate Magus 10 who possesses the feat Master Spellthief. He applies the ultimate magus class feature spellcasting and extraordinary ability arcane spell power to his wizard casting when he can and his sorcerer casting when he must. What's Dylan's caster level for his A) wizard spells and B) sorcerer spells?
The above prestige classes combined.
Ethan is a Bard 1 / Sorcerer 1 / Spellthief 2/ Wizard 3 / Green Star Adept 2 / Knight of the Weave 1 / Sublime Chord 6 / Ultimate Magus 4 (or another more favorable combination you choose) with the feat Master Spellthief. He applies everything he can to his wizard casting until he can apply it to his sublime chord casting and everything he must to his wizard casting or his sorcerer casting, favoring wizard. What are his caster levels for his A) bard spells, B) sorcerer spells, C) wizard spells, D) knight of the weave spells, and E) sublime chord spells?
I wish I could apologize for the complicated math necessary to answer this question. By the way, there's obviously no way to be sure, but I'm guessing that Mike McArtor and F. Wesley Schneider's intent was just to have the feat make one of their home games's Spellthief / Wizard a more productive party member, not to encourage absurdity. So, while I agree using the weird wording of prestige class's ability to advance casting in unusual ways possibly violates the feat's spirit, let's just assume it doesn't and that the Complete Scoundrel's authors knew exactly what they were doing when they wrote the feat Master Spellthief.