For me, this one is the most gamebreaking one. It’s happened to me fairly consistently (see my last post there – doesn’t seem to be limited to just building in spite of the title, which stems from the way the bug first showed up for me), and does not seem to have an obvious workaround (e.g., saving and loading don’t fix it), and it just makes your citizens stop working for the most part. (Mind you, the transparent buildings bit in the second post was just an aside, it may not even be part of the same bug. That one obviously isn’t such a big deal since it has such an easy and obvious workaround.)
The second would be this one, which is also annoying, but at least if you’re careful and you only place walls all the way around rather than drawn manually, or you leave places open in the floor plan for any internal walls you may build, you can avoid the crash.