Here is The Traveling Professor's list of the top 5 worst mistakes an international traveler can make:

Having No Travel Insurance:  the chance of losing luggage, having a trip delayed or cancelled, getting sick or even worse, needing to be evacuated from an overseas trip back home is slim.  However, the cost can be great.  See The Traveling Professor's comments about travel insurance in this The New York Times article by The Haggler.  Why international travelers don't have travel insurance is a mystery to The Professor, even though it can sometimes be obtained free, with the right travel credit card.  No matter what you do, at the very least have emergency evacuation insurance when traveling abroad.  

Taking a Ride from the Airport/Train Station to the Hotel when Solicited Inside the Lobby/Station: Some unscrupulous people prey on travelers arriving from a long, tiring trip with an offer for a quick ride to the hotel.  Don't do it or expect an exorbitant fee (or even worse).  Only take public transportation, use a registered ride share program, a shuttle or an official taxi into town.

Use a Money Exchange Booth:  Travelers are just asking to be ripped off with high commissions when doing so.  It is so easy just to take money out from an ATM or use a credit card whenever possible.

Book an Award Flight Connecting through London:  Taxes and fees for flights through London, even on award miles, can approximate the cost of purchasing a ticket outright with money.

Not Having the Right Travel Documents:  OK, everyone knows a passport is need to travel internationally, even to/from Canada.  But is a visa or ETIAS travel authorization  need to be obtained before departure? 

Join The Traveling Professor on one of his small group tours. Travel to Europe, South America and Canada with friendly and active adults, just like you.  Every tour is different, but travelers can expect solo travelers on a small group tour as well as accompanied travelers.  No one feels out of place on a Traveling Professor small group tour.