Ask HN: What was the last hard/time consuming thing you did?

I'm currently looking for a side project and was wondering what was the last hard thing you had to solve or did consume a lot more time than you expected?

Doesn't really matter too much what it was. I'm open for almost anything tech related. Setting up monitoring, analytics? Getting data from one point to another? Making sense of that data? Integrating or using service XYZ? Communicating with other teams? Communicating with customers?

I hope you get the idea :)

Thanks for your input in advance!