Once, years ago, while trying to compile a Pascal program, I wasted over an hour, trying to find out why the compiler was unable to find a subroutine that was obviously in my file -- right in front of my face!

The problem turned out to be a Pascal style { comment } many screens away was missing it's closing brace, like this:

{ comment number one

subroutine one

{ comment number two }

subroutine two
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So the compiler thought my code was commented out and I thought it was code! That was the day I started using syntax colors in my text editors. When a big section of code is colored as grey, the cause is obvious!