oraguy on Nov 13, 2018 on: Ask HN: What's the largest amount of bad code you have ever seen work?
Oracle Database 12.2.
It is close to 25 million lines of C code.
What an unimaginable horror! You can't change a single line of code in the product without breaking 1000s of existing tests. Generations of programmers have worked on that code under difficult deadlines and filled the code with all kinds of crap.
Very complex pieces of logic, memory management, context switching, etc. are all held together with thousands of flags. The whole code is ridden with mysterious macros that one cannot decipher without picking a notebook and expanding relevant pats of the macros by hand. It can take a day to two days to really understand what a macro does.