What I found were two incomplete multipolygons and two untagged ways. So much for the small error. 

I didn't delete any of your work, nor did I add a single new node.

What I did was that I added the members of one multipolygon to the other. I then deleted this multipolygon because it had become superfluous. I combined one of the untagged ways with another one, which deleted it. I completed the remaining multipolygon with the remaining untagged way and added the necessary tags for the Lac Vital. 