A very common mistake is the orientation of certain components. Diodes and transistors are the main culprits and can cause damage to themselves and other components if incorrectly connected. Transistor connections can be tricky with all the different case types, in particular TO92 types. For example, BC183 and BC183L have different connections. Check in component catalogs for pin orientations or click here for transistor pinouts
When using diodes for protecting transistors driving inductive loads (such as relays), be sure to check orientation carefully. They need to be reverse biased, if not they will conduct when the transistor switches on and usualy destroy itself and the transistor due to the large surge of current.
When inserting components into the PCB, do not force them if the holes are not large enough. Re-drill them to the required size. Most holes will be large enough at 0.9-1mm. For certain components like presets and PCB terminal connectors, a 1.2mm hole is required. Other components like PCB mounted switches and sockets, hole sizes can be up to 2.5mm.
Electrolytic capacitors can be dangerous when connected the wrong way round. If the power supply has the current capability, the electrolyte heats very quickly and the gas produced has enough pressure to propell the metal case many feet accompanied by a bad smell and a mess on the circuit board. This can be dangerous if inspecting the board at the time. TAKE CARE and always check the orientation of electrolytics, in particular the 100uF types.
Integrated citcuits are easy for students to insert incorrectly. Remember the dot identifies pin 1 and a divot in the case identifies the top. These can heat up and crack the case if inserted incorrectly and are almost certainly destroyed if not. It is always good advice to use IC holders.
Not so common but students can make mistakes with the resistor colour code and thus soldering wrong values in place. This is not usually a problem if the value is higher than required but if they are lower then damage to other components is a possibility. Check whatever is in series with the offending component and test/replace if circuit still refuses to work correctly.
If LED's fail to light correctly this can be due to resistor value too high or the LED incorrectly connected. Some of the leg lengths and flat faces that identify polarity can be difficult to see. Test the LED separately with a 330ohm resistor and a 5 volt supply. If an LED is connected without a current limiting resistor, it will almost certainly destroy itself within seconds. Short of this the colour of the LED will be altered and is best replaced.