A common mistake I do is to connect the power wires or battery the wrong way to a circuit. This results in reversed polarity and it may damage some components on your PCB or have even worst consequences.
The good news is that we can very easily protect our circuits from such errors by using a rectifier diode. It allows the current to flow in only one direction in your circuit. Pay attention to the direction of the diode versus the battery polarity.
The IN4000 diodes family is probably the most commonly used by hobbyists (see: this wikipedia page for a list of available models). Personally, I almost always use IN4001 diodes, since they are able to block a voltage of 50 volts while my circuits are usually powered with 9V or less. If you look at the diode, you’ll see there’s a line at one of its ends, which indicates the orientation of the diode. It represents the same line as in the diode symbol in the diagram above. On the picture, the diode is currently oriented in the same direction as the diode in the diagram.
When designing your circuits and sizing your power supply, you have to remember that your diode will cause a voltage drop of 0.7V. So, if you power your circuit with a 5V power supply but put a IN4001 diode to protect your circuit, your circuit will have only 4.3 available to work with, not 5V.