Where is it legal to bring weed across country or even state lines? There's no way to "forget" to remove something from your luggage that's universally illegal, unless you've choosen to flout the law because it's easy to get away with elsewhere.
You’ve never left something in a pocket before, or dropped something into an open suitcase without noticing it?
Maybe coins or random pocket trash, but never illegal drugs. I think I would notice 500 grams of weed, which is the minimum to receive the death penalty in Singapore.
Half a kilo isn't easy to overlook, I'll give you that.