In 1987 I wrote about 150 lines of 68000 assembler that applied a low-pass filter to every sound heard by the towed-array sonar system on the Los Angeles class fast-attack submarines. I was an intern (co-op) and nobody even looked at my code. But it fixed the problem that had been plaguing the ANN-BQQ-5D system, preventing it from going into trials and 'management' just shipped it once the tests started passing.

Compared to other things (including some of the ID3 tag processing code that's still in Windows today, and OLEView) this seems pretty important.