Sometimes we try to save changes we did using VisualEditor but instead get a pop-up with a "Parsoidserver-http: HTTP 400" error. This was once experienced by me, one of the students in our education program as well as by her teacher who leads the course, so the problem occurs quite widely.

The only chance is to go back to the VE and try to save again but this typically displays the same pop-up. In the end, you lose your edit and have to write it again.