Am I GUARANTEED not to ruin arduino circuit if I use this trick???????

Hello I have an arduino uno which outputs 5 volts and recommends a maximum current of 20mA
If I use a 250ohm resistor in series with any circuit am I GUARANTEED not to blow the arduino uno because V = IR and (V / R) = I and
(5 / R) < 20mA therefore R needs to be greater than 250ohms and then I am GUARANTEED not to go over 20mA

Is this a correct assumption or am I way off here?