It's not documented very well, but the C# compiler actually has 4 levels of warnings.  I like to think of them this way:

• Level 1 - These warnings are cases the compiler is 99.9% sure you did something wrong, but the code is syntactically and semantically legal.
• Level 4 - These warnings are cases where the naive user probably did something unintended, and we want to make sure they're aware of it.
• Level 2 and 3 are just somewhere in between.

This seems like a pretty reasonable set, although I've often wondered if 4 levels is the right number, maybe 3 or 2 would be better.  Anyway, often times we get requests from the ASP.NET team to raise or lowere the level of certain warnings.  Now the whole point of a warning is that it points out a possible problem with the code.  As such the compiler team has always mandated that we will feel free to add new warnings where appropriate, so those people who build with /warnaserror are just asking to be broken when they upgrade (there's a better way to do this and I'll get to it later).  Now back to ASP.NET, as anybody who has used it already knows, ASP.NET does not have a way of reporting warnings.  They don't go to a log or anything useful like that.  Instead they build with /warnaserror turned on and the warning level set to 1.  So all of our level 1 warnings become errors and break people's web sites.  Anybody see the problem here?

As promisied, here's the right way to use /warnaserror: on your developer machines, before submitting changes.  Then on your offical builds turn it off.  Keep a warning log and send it to the developers, but don't break your build because of a warning.  I don't know about your organization, but here a build break is painful!

--Grant