Avoiding the emotional stuff, the most intellectually difficult thing I ever did was figure out how floating point numbers were stored in Tandy Basic data files so I could convert them to Xenix MBASIC.
This was 1983. No internet, no Google. No help at the Library. I did it by brute force and some guided guessing. It was tough, but fun.
You can help the HubPages community highlight top quality content by ranking this answer up or down.