What are most of the mistakes that we make while planning a trip or what could be the things that we can miss?

Forgetting your passport, not bringing the correct currency, not confirming hotel reservations, forgetting to pack medication you need, not packing the proper clothing, not making the proper arrangements for mail pickup, lawn or driveway maintenance while you're gone. Not getting the correct vaccines. The list is long.

Over packing
Not having a small amount of foreign money when you land.
Not respecting their culture, so read and learn about it
Comparing it to your nation, believing your nation is best or better.