Two processes intend to add two to a number.
They each read the current value.
Then they each write back the value which is two bigger then the original.
If you instead use private fields and public getters/setters, or use actors to form a protective bubble around the mutable state, you get...
A race condition:
If you instead use private fields and public getters/setters, or use actors to form a protective bubble around the mutable state, you get...The exact same thing but with more boilerplate.