v1.9.1 included a way to stop the weird /_cache/app/content/content.php permissions issue (getting set to 000 instead of 755 or 777) but I've found that while it's mostly stable after this update, I have had a couple of drops. Has anyone else who has updated to this version noticed any rogue permissions issues?
Something to note: Good idea to have _mode: 0755 in your /_config/settings.yaml file as a fallback, if you don't already have it from the new template that came with 1.9.1.
Things to consider in my particular case: I wrote a plugin that empties the cache in an if...then statement, this might have something to do with the issue. The line looks like this:
if ( $var == TRUE ){ $this->cache->destroy(); }