I wonder what kind of issues one can get in distros that people prefer stable? What can break? For me it was once, years ago - X didn't start up, fixed in few

I wonder what kind of issues one can get in distros that people prefer stable? What can break? For me it was once, years ago - X didn't start up, fixed in few days after update. Most of breakages of Linux was my own doing (e.g. by mistake overwritting MBR). I use testing+unstable (I didn't know that backports exists, thanks) + self compile some software that doesn't exist in either or is too old. I also always compile kernel, prefer it to have less modules and configured towards performance on desktop.