Seriously, a simple bit of code you do that works perfectly fine in one module, can make shit break all over the place that don't even seem related.
Sometimes the compiler just fucks with you too. You do a compile for that module you just worked on and link it, and it fucks you over with shit that bat shit insane. Recompile the whole fucker and its fine.
lol, perl masterrace. I need ten open tabs and a manual to make out just what the hell all those shortcuts mean.
All the energy went into parsing BSP/SMD or thinking about engine structure (game frame, communication between server and client and so on). Using Unity or other crap like that has important benefit: you get "results" quickly and that can make you not get bored easily.