Seeing all the work for saving forms, maybe this is issue is unimportant. But since the 5.x-1.2 version is still the recommended version, I'm submitting it.
I ran into this with the wikitools module, that checks to see if the newly added title has the same nid as an existing one. When autosaving from node/add/wiki, a node is created. Then when I hit submit from the form, the wikitools validate thinks it's adding a new node, and node validate fails.
The problem is that on node/add, when the node is autosaved, it now has a nid, vid, and changed, and a different form action url. These need to be updated on the form, so that when it is submitted, it's submitting an edit instead of an add.