When exploring a new Savanna biome, I encountered a tree that had generated with the trunk in diagonal blocks
l x x
x l x
x x l
Like above, where x is empty space and l is a log. Didn't feel comfortable doing any poking around in code to see if this was an intended feature, but a search turns up no similar results in here or on the wiki.
I was obviously expecting the tree to generate with a straight trunk instead of a diagonal one. After taking a screenshot I proceeded to chop the tree down. Unfortunately I'd forgotten to use the default texture pack.
I don't really have a clue as to how this could be encountered. It was an oak tree on the edge of a Savanna and River biome.