The most "fun" I've ever had was decoding command and telemetry from piece of equipment for a ground station. The box would spit out this massive frame of data. It was a very long ASCII string that you would turn into binary to break into 6bit BCD values (no clue why they didn't use 4bit...). There were random flags of odd bit lengths (sometimes just a single bit, sometimes 5bits) thrown in between numbers for arbitrary reasons rather than just having all the binary flags up front. My python script was this ugly mess of slicing up the frame to turn it all into a very nice struct I could pass to the rest of the system. The manual with this piece of hardware was some old scan that must have been xeroxed a million times over so some portions of the document were just unreadable and you had to guess what those bits did. Other parts of the frame were just undocumented. Commands were send one by one as single letter with the actual ASCII representation of the numerical command parameter.

When I started the project, I looked online to see if anyone had done any previous work on this thing. A vendor was selling a GUI for the thing for $2000, I scoffed at the price and started working it myself. By the time I was done, it had probably cost my employer more than that but at least we had our own code that could connect to whatever you wanted rather than a GUI with a no API.