For me I would say it's fault finding. Releasing software is easy but finding the sometimes very obscure issues that crop up once released is always a challenge

For me I would say it's fault finding. Releasing software is easy but finding the sometimes very obscure issues that crop up once released is always a challenge, especially when bug reports are not always the most detailed. When dealing with some of these it's often necessary to delve deep into the internals of how something is working. I once spent 4-6 months working on a fix which turned out to be a stack corruption on a RTOS but was hard to pin down as the system usually took over 2 weeks to crash. I now mostly deal with embedded software so it can often be with conflicts of hardware which have never been tested together and with unpredictable results. I've also dealt with finding faults in software I had to maintain across several platforms/databases/etc written in a few languages. For the most part it's been fun or rewarding with some difficult times. Delving into the depths of a system does take some time but gives a wealth of knowledge that can usually be applied to another problem