Visa waiver / US immigration

This is really aimed at any check in or cabin crew, this is perhaps one of the most regular questions ( and heated arguments between the people who believe that the US has full access to our PNC etc. and the people who say the opposite) that pops up on any US travel forum - regarding criminal records and the visa waiver scheme - so many people must travel under the VWP without the slightest knowledge of whether they need a visa or not - just how often are people denied entry because they have knowingly or unknowingly travelled without a visa, in particular if they have lied and ticked NO on all the questions about arrests/convictions (and been found out somehow)?
I can understand if they have answered YES on the forms - I assume they are on the next plane home?
I would LOVE a definitive answer, one way or the other.
I can understand if they have answered YES on the forms - I assume they are on the next plane home?
I would LOVE a definitive answer, one way or the other.