US State Department says, “You will need sufficient proof of funds and a return plane ticket” for a tourist visit to every Schengen country I looked at. Something similar for several other countries. And I’ve often read folks warning others of this (or complaining of it).
But I've made fourteen entries into Schengen and thirty into other countries or regions and never been asked for either. (Actually more, but I didn't count transit, returning to USA, or port visits on ships.)
In most of those, I did not have a return or exit ticket. I know that border officers have discretion, but am I some sort of outlier, or are the outliers the folks I've seen/heard complaining about it?
I don't think the details matter, but for those who disagree:
Schengen 14 (es/pt/is/it/be/nl) México 10 UK 9 Canada 5 CA-4 1 (sv/hn) Corea S. 1 Iceland 1 Irlanda 1 Perú 1 Taiwan 1 Turquía 1
P.S. Yes, I am US citizen, but that’s not my fault. :-)