Don't run all network services on a Gentoo box.
Don't write yet another C++ strings library.
Don't write yet another C library. (that is, a libc implementation)
Don't put developer workstations on a network that connects to the internet.
Is there a story behind that one? That one is very common and not traditionally considered harmful.
It's especially important when the work product includes hazardous information. We had enough to cause large-scale disaster. Maybe I'd best leave it at that.
Even the smaller stuff is a problem. Imagine full data dumps from places like the Federal Reserve, Google (particularly Gmail and Project Zero), or Verisign.
Things can build over the years. At first it doesn't seem like a big deal to be on the internet. It's convenient. You don't have to buy two computers for everybody. Eventually, if you notice it or not, you may have a large amount of dangerous data.