Another thing that enabled this bug is that it was a language that allows misleading indentation.
Programmers indent all code. By making indentation to be not meanigful in your language you are ignoring large part of how programmers read and write code and allow for many misunderstandings between programmer and compiler.
Is there evidence that misleading indentation had a role in enabling the bug?
You can make a typo, you can duplicate a line, you can misindent a line, you can put a brace in the wrong spot. I believe different languages simply trade forms of potential mistakes for other forms. Ultimately no language will magically fix or make bad code known; it has to be detected somehow. Tests, static analysis, manual review, whatever.
You are right. It wasn't the case for this exact bug. The source of this bug was probably the the design of the code editor that had function that duplicates line bound to a single keyboard shortcut right next to shortcut for saving file that every programmer uses roughly 1000 times a day. (What for? What's wrong with Home, Home, Shift+Down, Ctrl+C, Ctrl+V, Ctrl+V? Do you really need to duplicate single line in place that much so you need single key combination?).
However language that doesn't ignore indentation would make such error benign as duplicating the line wouldn't place code in completely different level of AST.
All programmers indent. Why so many languages happily ignore that?
I'm a huge fan of the Go compiler's general stubbornness. Have unused imports? Compile error. Have unreferenced vars? Compile error. Perhaps misleading indentation should be another compile error.
Pain the ass? You bet; it's a feature. Ignore the whiny kids who insist their 'flow' is broken by having to insert semicolons. Most software work is maintenance, not new code.
"All programmers indent. Why so many languages happily ignore that?"
I've encountered places where isolated breaks in indentation style radically increased readability. Unfortunately, I can't recall them well enough to reproduce here or know whether I'd now have a better solution. It surprised me at the time.
Python doesn't allow misleading indentation in the specific case at issue, but does allow misleading indentation (e.g., inside list comprehensions split over multiple lines).
It's optional though, and leaves too much room for ambiguity IMO, which I find annoying since the point of significant white-space is maintaining consistency.
These are all equivalent:
main = do
something
main =
do
something
main =
do something
And obviously, you can also not use `do` notation.
Programmers indent all code. By making indentation to be not meanigful in your language you are ignoring large part of how programmers read and write code and allow for many misunderstandings between programmer and compiler.