Frequently we're given the advice "just be yourself". And yet, this is--just as frequently--the wrong advice.

When should you not be yourself? When has it gotten you into trouble? When has it worked out great?

I think we can stick with the common definition of "yourself", which is "how you behave most days" or "how you usually behave in those situations". A friend of mine once pointed out that you're always being yourself, since you can't be someone else. True, but not what I'm interested in.

Comments? Examples? Refutations?