While I agree it's on traveler's responsibility to ensure they have the proper entry document, I do think honest mistakes happen and it may be more frequent than you think when a border officer exercises their discretion.

I once read a UK foreign student who took the ferry to Ireland not knowing he needs a separate visa to enter Ireland. No one checked his visa before boarding the ferry. He was allowed in based on the good faith of the border officer, after he showed a one night hotel booking with ferry ticket coming back to UK the day after.