Several abilities including feat Extra Spell and the Master Specialist class feature Expanded Spellbook allow spellcasters to learn an additional spell. However, these abiities do not specify that the spell learned must be on their class spell list.
The reason why I feel that spells outside the caster's normal class list would be permitted in the case of Expanded Spellbook, at least, is that gaining a small number of spells into a wizard's spellbook is not really a terribly useful ability; a wizard can do that pretty easily anyway. Also, it makes a lot of sense, flavor-wise, if a wizard who is an utmost master of his school of magic can cast a handful of spells that other wizards can't.
If a feat, class feature, or other ability says that a spellcaster can learn a new spell, and that ability does not specify that the spell must come from their class list, can they learn spells that are not on their class list? I'm specifically asking about D&D 3.5.
Best Answer
Extra Spell is specifically addressed in the FAQ (p. 40):
This is the general rule -- you have a class ability that lets you cast spells from a particular class list only. For instance, the wizard has the following ability:
For something to give you spells not on your class list, it has to explicitly call this out as an exception to the more general rule. The Master Specialist ability is worded almost identically to the wizard's regular spell book entry, so it clearly does not allow this:
If it isn't on your class list, how would you even determine if it was a level that you can cast?