Originally posted by mrbluetone
Excellent...I didn't know there was such a reference index, let alone that I was playing them by accident!
That's no accident. There is basically no way (except resigning before move 1) to not play an opening from ECO. Even if you do something daft like 1. f3, that's covered under A00. And if you play anything even remotely reasonable, it will have been played before and fall under one opening or another - possibly a rare one, but a known opening with a non-A00 ECO code nevertheless.