回复：父母回国I-94表未交，如何补救？

Next time you have difficulty to prove when your parents left the US. US government does not know when they left the country so they may have problem to obtain a visa next time. The only thing can be done is that keep the original airline ticket which has flight information on it. Again even with this, everyone can buy a ticket (with flight information on it) without using it. In a word, it is a trouble.