Five of them, and I have seen all five in a single game more than once. None involve any deep understanding of chess. They are all cases of a player doing something that feels active while their opponent quietly gets three pieces out.
The five
- The queen comes out. 1.e4 e5 2.Qh5 hopes for mate on f7, and against 2…Nc6 and 3…g6 it just loses time; there is a page on answering it. Do watch f7 while you defend, mind: after 1.e4 e5 2.Qf3 Nc6 3.Bc4, the careless 3…Nd4 allows 4.Qxf7 mate.
- The same piece, over and over. 1.e4 e5 2.Nf3 Nc6 3.Ng5 Nf6 4.Nf3 Bc5 and White has made three knight moves to arrive back on f3, while Black has a knight, a knight and a bishop out. Three moves for nothing is a bigger deficit than most beginners ever recover from.
- The greedy pawn. After 1.e4 e5 2.Nf3 Nf6 3.Nxe5, the natural 3…Nxe4 runs into 4.Qe2, and now 4…Nf6 loses the queen to 5.Nc6+, a discovered check that hits d8 on the way. The move is 4…Qe7, holding the knight, and after 5.Qxe4 d6 Black gets the piece back.
- The king that stays on e1. See the board below. This is the expensive one.
- The f-pawn. 1.e4 e5 2.Nf3 f6 opens the e8-h5 diagonal at the king, and 3.Nxe5 fxe5 4.Qh5+ is already winning for White. The f-pawn defends f7 and f2; move it before you have castled and you are opening a door in your own wall.
The king that stayed in the middle
8 
7 
2 
a 
b 
c 
d 
e f g h 1 8 7 2 a c d e h 1 Black took the queen and got mated two moves later, and the reason is written on the board: the king on e7 is where the king started, more or less, in a position with four white pieces developed. Black’s rooks never moved.
That is the whole lesson. Castling is not a formality you get round to; it is the move that makes every other mistake survivable. Grab a pawn with your king on g1 and you might just be a pawn up.
If you want the cure rather than the diagnosis, the opening principles page is the checklist, and not blundering is the habit underneath all five of these.