For me the hard bugs are the multi-threading ones where I made a bad assumption about order of execution. One from 30 years ago that I still remember is when a

For me the hard bugs are the multi-threading ones where I made a bad assumption about order of execution. One from 30 years ago that I still remember is when a "packet received" interrupt for a response came in before the "packet sent" interrupt of the request, due to queuing in the lower levels of the network device driver. It totally crashed our system because the pointer pointing to a transaction data structure wasn't initialized yet. It caused a triple fault so it was an instantaneous reboot. Since then I've become more defensive with classic multithreading.