Last week I left you with this position from the Four Knights Game.
Black has several reasonable moves here: 8… Be7 is most natural and the engine choice, but a6, for example, would also be fine.
But 8… d6, which looks plausible, would be a fatal error after 9. Nc6 bxc6 10. Qxc6+ Bd7 11. Nxc7+ Ke7 12. Re1+ Be6 13. Nxa8
Very easy to miss over the board.
This was a position from the lichess.org tactics trainer: in an online game Black played d6 and White missed the refutation.
Not so hard to solve, though, if you know there’s something there.