Generalization
The biggest, and perhaps the most common mistake made is perhaps generalizing. Stating “All bananas are yellow” will only result in you being proven wrong, for most bananas are green at some point, and some turn brown at a later one.
When a human generalizes, it typically betrays two things about them.
1. They think they are undoubtedly clever.
2. They lack some sense of self-awareness.
The problem when generalizing is when you lack the data. You could say “All bananas I have observed have been yellow” to far more accuracy, or you could say “Most bananas are yellow” and be accurate there as well. But an absolute is a dangerous thing.
The problem becomes especially apparent when applied to humanity. Making a broad, sweeping statement that defines a large group of people is an easy way to make mistakes.
Stating “All muslims are terrorists” would be stating that 2 billion people seek to use violence to further an agenda. Were that the case, the other 6 billion of us would probably experience more firsthand terrorism.
However, stating that “Islam is a religion of peace” makes the same mistake. Were 2 billion people unwilling to harm others, then the many attacks suffered within Europe and Western Asia would not be present.
You cannot define any group of people by generalizing them, not even if its positive. Ultimately, humanity is an incredibly diverse species mentally, and something as simple as whether or not we will take a life is bound to have more answers then “yes” and “no”.