Check your work.
It seems to be a stupidly simple piece of advice, yet it's amazing how often I run into problems from a simple lack of looking. This is QA 101, and as with a lot of basics we learned earlier in life, we've forgotten them as we get more sophisticated.
I don't know how often people assume that they wrote their code correctly, so things should work and they don't bother looking at the page in a browser. I just ran into this with one of my programmers who moved a site, and forgot to include the file that told the server what to do with the code. The whole site was printing source code, not pretty HTML.
Then there was a more subtle problem I saw with incompatible browsers. We had a programmer who wrote some nasty things about the client in the code thinking it will never show up. Except it was in the source frameset, and the client visited the site with a browser that didn't support frames (they still exist).
And this isn't just a technical issue -- turn on your spell check before sending an email but be sure to READ the email before hitting send. We wrote a proposal for a warehouse system, and the guy accepted the spelling correction blindly. Apparently we ended up bidding on a whorehouse system instead.
Fortunately we generally check each other's work at Conquent. It's better to find these things out before the client does, but it's even better to find it or for yourself before publishing it.