What was the worst bug you've ever solved ?
earl: The most expensive bug that I can talk about publicly that I've encountered:I used to work on software for a very expensive (started at 70k) DNA/protein analysis hardware solution.  This was in the late 90s, and our GUI was a couple million lines of MFC code.  I was responsible for an analysis package written in cross platform (linux and windows) C++, but the main UI and all the complex interface was MFC.My then boss was a unix idiot who hated Windows.  Which is fine and all, but a Windows product was paying our salaries.  So one day he decided to rewrite my (well tested, though not unit tested) file handling code written using the win32 API and port it to posix.  Not to accomplish anything different, mind you, and not to add it to the cross platform bits since it was useless in isolation and, oh yeah, we had multiple millions of lines of MFC so nothing was ever getting ported.In any case, during this "upgrade", he found a function called DirTouch, which was intended to make sure certain directories existed during the normal course of saving data.  Well, this gentleman subtly changed the semantics from "create the directory if it doesn't exist" to "create the directory if it doesn't exist, but if it does already, then silently delete whatever is there".  This wasn't the whole bug, but this change of semantics to a destructive function was the root cause.This change got shipped.  One of our customers killed literally more than $1.3MM worth of data, since each machine run might cost $100K given lab time, reagents, prep, etc.