ECO itself is not error-free, as the following example illustrates. After the opening moves 1.e4 e6 2.d4 d5 3.Nc3 Nf6 4.Bg5 Be7 5.e5 Nfd7 6.Bxe7 Qxe7 7.f4 O-O 8.Nf3 c5 9.Bd3 f5 10.exf6 Qxf6 11.Ng5 Qxf4 12.Bxh7+ Kh8 13.Qh5, Keres and Euwe give 13...Qf2+ 14.Kd1 Nf6 15.Qh3 e5 16.Bf5+ as equal.
I suppose that they did this because White can now force a repetition of position with 16...Kg8 17.Bh7+, etc. One evening, I sat at my chessboard for fifteen minutes trying to figure out how Black draws after 16...Kg8 17.Bxc8.
The answer is: he can't!
{This article originally appeared in Atlantic Chess News in 1990}