CJS Purdy said a number of important things during his career, but the one that sticks most with me is "Look at all checks and captures". This is the most common piece of advice I give to new chess players, and the most common cause of my blunders, when I don't follow it myself.
To reinforce the point here are two quick games played in the last couple of days, and in both the losing side probably failed to look at all (as opposed to some) checks and captures.
(Note. I'll leave the names off the games to protect the victims)
1.d4 d5 2.c4 e6 3.Nc3 c6 4.Nf3 Nf6 5.e3 Nbd7 6.g4 Nxg4 7.Rg1 h5 8.h3 Ngf6 9.Qc2 Bb4 10.Rxg7 dxc4 11.Ng5 Rf8 12.Nxe6 fxe6 13.Qg6+ 1-0
1.e4 e5 2.Nf3 Nf6 3.Bc4 Nxe4 4.Nc3 d5 5.Bxd5 Nxc3 6.dxc3 c6 7.Bxf7+ Ke7 8.Bg5+ 1-0