1. non-reproducable, long run with unnoticable defects on production system.
2. design a new system that considering requirments from many stakeholders.
I worked on a Interstate Construction long ago and because of weather and other difficulties we run out of time and have a deadline to deliver the project or face heavy penalties. I worked as a superintendent and spent 96 hours (4 days) without sleeping or resting until we finalized the project so the government of both Countries could maintained their meeting schedule with the European Union inspectors ( making sure the funds were really used to execute the project)
They were difficult. But noticing that the member was not capable to build the device before we enter the lab, or persuading the key QA not to quit at that moment, is much more tougher for me. May be I cannot make it for my life.
So what difficult for me may not be difficult for you. :-)