I'm not sure you're familiar with true pain in your arse then, because it's way beyond that. Depending on the previous coder, it could be akin to trying to ram a tree trunk up your arse. Keeping functionality in refactored code can be an extraordinarly laborious project.
Yeah, I once tried to re-engineer someone elses mod (Quake Wars frmo Quake2, fyi). I'd get several months work in, make improvements, and then hit a wall with buggy/unreadable code and have to start all over. I'm not a great programmer, though.
Usually I find myself swearing a lot and yelling things like 'DOES THIS ACTUALLY HAVE AN AUTHOR OR DID SOMEONE MIX UP A BUNCH OF VMFS IN A BUCKET?' WHY IS NOTHING ON THE GRID? WHO THE HELL MAKES SOMETHING 37 UNITS WIDE? WHY 37? WHY NOT 32 OR 48? OH GOD EVERYTHING IS FUNC_DETAIL WHY IS ALL THE VISBLOCKING GEOMETRY DUPLICATED INTO AN OCCLUDER? WHAT DOES LOLBUTON1 DO TO OFMGRELAYB? WHY IS WTFBBQTRIGGER INVOLVED? WHY DO THEY HAVE SUCH STUPID NAMES? WHY IS THERE A FUNC_FISHPOOL THE SIZE OF THE MAP IN THE SKYBOX?
lolwut func_fishpool? My personal gripe in coding is magic numbers. They're just so magical. Also, lack of comments detailing exactly what this function (or block of code if the moron isn't using modular code) does.