1. Create a function that returns a value. Mark down the variable.
2. Create a function that uses previous function created.
3. Try to use the 1st function's returning variable inside 2nd function, it will be defined and overwriting any value with same name.

(2 Examples are provided in Additional Information)

Example 1:

fnc_getVariable = { 
  _var1=5;
  _var1
};

fnc_mainCode = { 
  _var1 = 3;
  _var2 = [] call fnc_getVariable; 
  _varResult = _var1 + _var2;
  systemChat str _varResult
};
[] call fnc_mainCode; //returns 10 (Expected 3+5 = 8 instead)

What happens in here is that _var2, as expected, getting value 5 by fnc_getVariable, however it also overwrites _var1 because fnc_getVariable returns variable named _var1 , and also overwrites _var1 inside fnc_mainCode.

Example 2:

fnc_getVariable = {
  _var1=5;
  _var1
};

fnc_mainCode = {
  _var1 = null;
  [] call fnc_getVariable;
  hint str _var1;
}; 
[] call fnc_mainCode; // Returns: 5 , Expected: nothing.

I guess this example shows the problem better, I don't know if this is intended behavior or not, but it really looks like it shouldn't be as it is confusing and I personally found this unexpected behavior and fixed my code in a very long period.

I also apologize if category is unfitting, it looked like the most fitting.