It came up in my last campaign that the wizard came across an arcane scroll of Silence, and wanted to copy it into his spellbook. But it isn't on the wizard spell list! It's a bard/cleric spell. The rules say nothing about whether a wizard may copy arcane spells into his spellbook, so do I permit it, or not?
In fact, reviewing them now, it looks like the rules as written don't even specify that the scroll to be copied must be arcane! It seems preposterous that a wizard could copy a spell from a scroll he wouldn't even be able to use, so it seems uncontroversial to say that the scroll to be copied must be arcane.
But that doesn't solve the problem. The scroll is arcane, can the wizard copy it? For aid, I turned to the archivist, a standard class from Heroes of Horror. The archivist is clearly intended to be the divine counterpart to the wizard. The archivist casts spells from a prayer book. He automatically learns spells from the cleric list every level, but he can copy spells into his prayer book from scrolls or other prayer books, just like a wizard.
Most relevantly, the description of the archivist specifies that he can copy any divine spell into his prayer book, even if it isn't on the cleric spell list. The archivist can learn a druid spell if he comes across a divine scroll. He can learn domain spells, if some clerics scribed domain spell scrolls, even spells that aren't available in divine form outside of domains. The rules are very clear: this is exactly how the archivist is intended to be used.
So, if the archivist is intended to be able to copy any and all divine scrolls, and the archivist is intended to be a divine wizard, that inclines one to suspect that the wizard is intended to be able to copy any and all arcane scrolls.
On the other hand, as in the second paragraph, it does seem preposterous that the wizard should be permitted to copy a spell from a scroll he wouldn't actually be able to use. To be able to cast an arcane scroll that isn't on the wizard spell list, the wizard needs to make a Use Magic Device check. Notably, Use Magic Device is not a class skill for the wizard, and it cannot be used untrained.
Eventually, I solved this problem by declaring that a wizard can copy non-wizard scrolls, but while doing so, he must pass the same Use Magic Device check that he would need to pass to be able to use the scroll at all. At which point, the wizard raged slightly because he had no ranks in Use Magic Device.