There are 3 common mistakes students make when soldering.
Using the soldering iron like a paintbrush or dripping solder.
This method fails for 2 reasons. It uses contaminated/ not fresh solder. The wires/leads and pads do not get hot enough for the solder to effectively wet them. This means your solder joint may not produce a good electrical connection. We work too hard producing good projects to have them go bad from a bad solder joint.
Putting the solder on the pad before melting it.
This method may not heat the wires/leads and pads enough for the solder to wet the joint.
Heating a part too much. Students may trying for too long to fix a bad joint which ends up heating the component too hot and destroying it internally. Before this becomes a problem try to come up with a plan to fix the problem you have. There are some characteristic signs you can look at to tell you how to fix your problem.
Pitfalls