I'm here thinking, "Hello. -1 signed number, string::find() returns a unsigned, I'm supposed to get a unsigned/signed comparison warning here."
But now that I think about it, I think I'd like a super strict warning mode. Let's call it /W36. I want this mode to warn me about widening conversion, assignment between variables of different typedefs, etc. This is mostly to catch potentially non-portable code that relies on certain typedefs being used for STL implementations, etc.
Maybe what I really need to do is drag my "barely standard" STL project back from mothballs. It's your typical annoying template library: only things that are required to be primitive types are; everything else is a class type. Also triggers assertions at a drop of a hat. Basically it tries to define undefined behaviour as being either non-compiling or crashing the system. I only really worked on the vector implementation, but it was a doozy. Iterators worked on a observer pattern, so when a reallocation occured, they actually set a invalidated flag so they could assert if you tried to use them.