I noticed several things about players when they play. Usually people always check their opponents graveyards to see if a card has been played or not, or to see how many pieces of a certain card your opponent has left in his deck.
But many people always forget about the removed from play zone. If you watch duel videos, you can notice that many people never touch the banished zone.
Sometimes maybe you forgot your opponent has a Veiler or a Damsel banished from BLS. Forgetting Veiler could end up with a mistake of calculating probability (anyone can do it, with college math), and forgetting Damsel is banished could get you in trouble with Leviar.