I came across this interesting looking leaf spot during a short walk recently.
Here is is again, on a different leaf from the same tree (a Chestnut Oak).
This is the kind of thing that we got calls for all the time at the Master Gardener Hotline, where I volunteered this past summer.
But without all the Cooperative Extension's plant books and other resources, I'm at a loss. (And this is one of those occasions where Google brings up too many choices to be useful.)