I find “co-ordinating distributed transactions” to be very hard. Getting any kind of optimum cooperation between self-centred agents is tricky.

Also — “Jevon’s paradox”. That one is nasty! For example: just about anything we do to decrease use of fossils fuels by some small percent, makes the corresponding process more efficient, more profitable, and thus happen more. That’s a nasty nasty problem. I guess it’s not specific to computer science, but all engineering.