Hypertext pioneer Scott Johnson recently wrote a lively rant on software engineers who don't test their own code sufficiently. As a manager, this drives him up the wall: you tell someone to fix a bug, they say it's fixed. You take a look and the bug's still there.
He thinks this is laziness, arrogance, and irresponsibility. Sometimes it is. But sometimes, I think, it's really another familiar problem: editing your own work is often exceedingly difficult. When you know what something says, your senses may perceive what you expect, not what's actually on the page.
Johnson recalls how a missing apostrophe cost his company $1312 one afternoon. Proponents of agile development argue that systematically designing software to be self-testing turns out to be cost-effective for exactly this reason. The test system for Storyspace 2 was a big win, and improving the Tinderbox test manager is a major priority.