Back in the 1970s, when I was just a high school kid reading computer magazines, I saw references to a study (I believe by IBM), that said in production code, that 70 or 80% of the lines of code was error handling; only 20% or 30% was the "golden path". I'm not sure I saw an actual reference, even then, and I certainly cannot give a reference now.

Does anybody know the study in question? (I have never seen another study on the topic. Does anyone else know of one?)

This was almost certainly either done in assembly or PL/I or Algol or something. Do more modern languages change it? Exceptions? Dual-track programming (options or Maybes)?

Regardless of exact numbers, yes, error cases are something you have to think about all the time.