Usually, BD ends his updates with a few lines about development. Those lines are written in what Ren'Py calls "label", a specific portion of script. But that ending label is deleted in the next update and replaced with new content. So if you save you game in those lines, next version won't have that label and you won't be able to load that save because save/load system works that way: Ren'Py first looks for the exact line where you save your game and, if it doesn't exist anymore, it tries to rollback to a previous line within that same label. As in WVM that ending label is always deleted from one version to another, game simply breaks as Ren'Py can't find anywhere to go. So NEVER save your game during those messages