I've converted literally hundreds of mobis to epub and ironically for the very first time today I've had a hang/whatever. I came in here to post about it.
After upgrading to the latest version of Calibre today, I downloaded a few free books from Amazon, including Separate Truth by Michael McManus. The latter just would not finish converting. If I double clicked on the book in Calibre (still only showing format as mobi) it opened in the epub viewer, so it HAD converted, it just wasn't being finalised. Anyway, I tried several times with fresh downloads etc. and eventually it converted in a few seconds as though nothing had ever been amiss.
Go figure.