>what are they supposed to do when there is no memory left
Well on Linux they are apparently supposed to return memory anyway and at some point in the future possibly SEGV your process when you happen to dereference some unrelated pointer.
You can tell Linux that you don't want overcommit. You will probably discover that you're now even more miserable and change it back, but it's an option.
Whenever I switch off overcommitting, every program on my system (that I'm using) dies, one by one, over the course of 2–5 seconds, followed by Xorg. It's quite pretty.
Well on Linux they are apparently supposed to return memory anyway and at some point in the future possibly SEGV your process when you happen to dereference some unrelated pointer.