I spent quite a while testing, but I couldn't figure out exactly what I did to trigger the Unable to parse content due to a Parsoid failure error. I'll give you as much of a report as I can. It started out with this text:
I wanted to thank you for adding a 'Discussion moving to' summary link to the closed discussion. It is ''greatly appreciated.
Note the '' before the word greatly, where I tried to add italics in visual mode. I then switched to Wiki mode, and I thought it odd that Flow put the nowiki tag starting at the apostrophy before the word Discussion. So I made a randomish edit... no clear plan in mind.... I don't recall exactly what I did.... it's possible I left an un-paired nowiki tag. I just wanted to see what Flow would do when I flipped back and forth between Visual and Wiki modes. Each time I flipped to visual and back, Flow re-wrote it differently each time. Flow was applying an unstable round-trip transformation. After about 4 round trips it crapped out with Unable to parse content due to a Parsoid failure.
It's a serious design flaw that Flow constantly translates and translates the content, with no underlying "fixed reality". This is why it keeps mangling stuff. With Talk pages, wikitext is the fixed reality, and it simply gets rendered.