At Singapore Airport, there is a huge sign above the immigration gates announcing that drug possession carries the death penalty. Every time I pass through I start questioning how sure I am that I don’t have any drugs on me. (I don’t use drugs or live with anyone who does, and packed my bags myself and didn’t leave them out of my sight, which is about >99.99% certainty, though that >0.01% bears a heavy weight.)
And even if not, how do you know you didn’t step on a tiny yet detectable grain of hashish on the way to the airport? Someone ended up in jail in Dubai for “smuggling” such a grain on the sole of their shoe.
The times I've gone to Sibgapore that sign makes me mentally go through my luggage, and I'm immediately reminded that I'm smuggling an obscene amount of snus (for personal use). It's no death penalty, but it's a pretty hefty fine.