Maybe not worst, but most frustrating. One of the systems I helped maintain at my first job had a few different bugs that caused bad data in the database. The "solution" to the problem was to write SQL queries to directly fix the production data. This would take one member of our team (it rotated weekly) about an hour every day to fix because there were literally dozens of these errors.

All the devs knew that we could identify the root cause and fix it in, probably, 3-4 days tops. Management would never approve the time because it would take longer to fix the root cause than it took to fix the data.

I worked at that company for 7 years. The bug was there when I came on, and it was there when I left.