I once had to write a feature, which should allow the user to login and edit an appointment, which was automatically set. All the data we got, came from an incredibly unreliable API. And with incredible unreliable I mean like heisenbug-level unreliable.
The API spoke perfectly unreadable xml and was a horror to work with.

After a few weeks of me being messed with by this shit piece of an API, I finally got something which did kind of work sometimes.
Proper error handling has been added later and just before I was done, fixing all the flaws of their data management and nonsense status codes (not http status codes) which rarely correlated in at least some way with their data, our client said "scrap this, we don't want it anymore"

Many hours and effort gone, this thing worked almost perfectly.