The difference is that a bug implies that it is an outside influence, a defect is something that has to do with making the thing in the first place. A bug you can see as happenstance, a defect implies liability. Huge difference.
I guess I've over-corrected in the other direction. All of the "bugs" I've ever dealt with were caused by engineers, so perhaps the original meaning (similar to "glitch"?) was lost on me.
The word comes from an insect that got caught in the wiring of one of the earliest vacuum tube computers. This caused the thing to malfunction and removing the insect became 'debugging'... the rest is history. I think it's a fun term but defect is far better for the man made software issues.
