Biggest sin? I was working in NYC, and my employer wanted me to document a package we'd bought from a British bank. 1979 was pre-Web, but not pre-e-mail. When I was told I'd have to spend a month in London interacting with the developer, I didn't say "You know, I could do the documentation right here, and just e-mail him my questions".

Biggest pit-fall? I implemented a communications emulator for an HP midi to act as if it was an IBM 370. I later found out HP had been contracted to develop it for United Technologies, which needed it to test a robot tank they were developing...