What was the worst bug you've ever solved ?
JeffJenkins: Non-strict comparator with the STL.  Nearly impossible to identify when it's happening.  It's happened once to me and once or twice to a coworker over the last couple years, and takes 3-5 days to debug every time.Example:  struct FooSort {
    bool operator()(Foo const& a, Foo const& b)  const {
  -    return not a < b ;
  +    return b < a;
     }
  };