Several years ago I got caught out on arrival at Orlando. The US Border officer asked to see my ESTA. I had forgotten to print it and being before smartphones I didn't have an electronic copy. I assumed that the US border force would be able to check their systems but was advised that some US entry points do not have electronic access to the ESTA system, Orlando being one of them. I had to use all my powers of persuasion to be let into the USA. Fortunately I had flown into the US several times in the recent past including ORD the month before and could show the entry and exit stamp for that flight. After that experience I always have a physical hard copy with a back up pdf on my phone.