You only get one move in a row

While it may seem obvious that after you move, your opponent gets a move, I suspect the root cause of a number of blunders is due to ignoring this fact. At the junior level analysis that reads "I go here, then my opponent does nothing, and then I go there and win" is legion, but I've been guilty of it as well.
Here is a famous games where the colourfully named Fedor Ivanovich Dus Chotimirsky would have done well to remember that his opponent is allowed to move in reply.

Marshall,F - Dus Chotimirsky,F [D21]
Karlsbad Karlsbad, 1911

1.d4 d5 2.c4 e6 3.Nf3 dxc4 4.e3 a6 5.Ne5 Nd7 6.Nxd7 Bxd7 7.Bxc4 Bc6 8.0-0 Bd6 9.Nc3 Qh4 10.f4 Nf6 11.Bd2 Ng4 12.h3 Qg3 Now Marshall is faced with mate on h2 and g2. But what reply did Dus Chotimirsky overlook?