When people rip you off, cheat on you or lie to you; they're not acting uncharacteristically.
Nobody can act uncharacteristically.
When they do something, its indicative of their character, which is revealed by their words and actions.
"That's not like him", or "She's not really like that" aren't observations of a person acting outside of their personality, since that's impossible.
Everything one does is in character. Everything they say comes from who they are.
When they behave in ways you didn't predict; its not because they aren't acting like themselves, its because you're a bad judge of character, who didn't correctly anticipate what they're capable of.
Maintaining the belief that you're a sophisticated connossieur of human nature, in spite of constant misjudgements about your acquaintances takes an incredible level of self delusion.
If only they'd consult you beforehand, they wouldn't make the mistake of accidentally not being themselves.
After all, you're a better judge of who they really are, deep down, than they are.
When they do horrible things, its because they don't know themselves like you do.
Unless you stop to ponder this, you'll keep considering yourself a discerning individual of impressive insight and uncanny wisdom, who is occasionally disappointed by self ignorant friends, family and co-workers.
You won't learn to judge the character of others until you own your misjudgements.
Its not them, its you.
Stay humble, and have a fantastic day.