I couldn't figure out a way to import transactions sanely into Yayoi Kaikei (the Quickbooks of Japan) to satisfy Japanese requirements for double-entry bookkeeping, so I implemented double-entry bookkeeping up to Japanese GAAP with about a dozen spreadsheets. It had to reconcile with information I got from a half-dozen systems, in varying formats. Also, most of the information coming in was in dollars, English, and either PST or GMT -- all three of which would get a look of disapproval from the Ogaki tax office -- so there's extensive use of historical lookup tables to massage the data to meet requirements without having to pay a bilingual bookkeeper to manually process everything.

If you add in the one-and-done Ruby scripts that I made to grab the information from the various computer systems and then spit out CSV for copy/pasting into Excel, this is unquestionably the worst bubble gum and duct tape programming job I've ever had the misfortune of participating in -- but it let me submit my tax return in time for the deadline, with numbers that looked pretty reasonable.