There are three simple reasons for throwing the BS Flag:

• Lies or mistruths in any form (full, half, overt or by omission)
• Any instance or instances of injustice (not "fair")
• Sub-optimal actions or methods (relative to some objective)

If you are skinning a cat, and I know a better way you can do it, I can throw the BS Flag. Criterion number three is at play. If you lie to me or leave out some detail in a half-truth, I can throw the BS Flag. So what's the rub? You guessed it: ambiguity and difference of opinion. Well welcome to the world and the story of history!