Thanks guys for all the great comments about accidentally taking another guy’s loaf of bread. I share all your intuitions, and I definitely should have realized the parallels to accidentally kicking someone you didn’t mean to kick, or accidentally eating something you didn’t mean to (being a vegetarian myself). So let me ask a follow-up, which will lead to why I asked the first question.
Question 2: could you describe the situation as Sal accidentally took a loaf of bread?
I think not. Without anything in the sentence to contrast the real state-of-affairs with Sal’s mistaken view of the world, “accidentally” doesn’t work. Everything in the sentence is consistent with what Sal meant to do.
The first thing this means is that accidentally is one of those adverbs that creates a non-monotonic context. That is, normally Sal took Tom’s loaf of bread entails Sal took a loaf of bread because a loaf includes Tom’s loaf. But when you add accidentally, the entailment stops working (assuming my intuitions about Question 2 are correct).
Now, why did I ask the original question? It seemed that in many “accidentally”-sentences, there are alternatives: Jack’s ass vs Sal’s in Erik’s comment. If there were two loaves of bread, then there would be an alternative: Sal took Tom’s loaf instead of his own. But in the situation I concocted, there was really only one loaf: just different ways it could relate to different people. Sal didn’t take one loaf rather than another; he took a loaf thinking it had property X when it really had property Y. And that’s enough, I suppose, to license the description “accidentally.”
So then there’s another question. Let’s say Sal walks through Tom’s property, and let’s say that (for some reason irrelevant to us) that is illegal. But Sal doesn’t know that. Did he accidentally walk through Tom’s property? Where Sal thinks Tom’s property has attribute X (legal-to-walk-through) when it has attribute Y (illegal-to-walk-through).