At university (a long time ago) on my first excursion with the Modula II compiler on the Vax, I accidentally inserted a # as the first character in a long source file. The compiler dutifully reported every line in the file as being in error.

I was inexperienced at reading compilerese (and source control and diffing were not commonplace at that time) and it took me a good day to extricate myself from my bewilderment.

I'm still mildly amused that such a small change to a more-or-less working codebase produced such generous output...