I’ve heard that sometimes ordering decaf will get you a caffeinated drink and that a lot of places disregard or don’t pay enough attention to that particular request. I have no source or any numbers though.

It definitely varies wildly. Some methods of decaffeination might leave as much as one-third of the coffee still in the beans.

I'm a regular coffee drinker (and not caffeine sensitive), and I do get a placebo boost from decaf, but without any noticeable effects from actual caffeine.