This happened to me as well.
Was playing 8-ball scotch doubles. My turn at the table, I just stepped up to bat, zeroed in on the next ball and fired it into the corner pocket, fell on the next object ball perfect when my friend pointed out I was using the 9-ball as the cue ball.
Bonehead move.
In my defense however, the cue ball we were using was an ultra stained yellow measles ball, almost the same yellow as the 9-ball stripe.
But does that mean my concentration was just so peaked, I was in ultimate focus on the object ball, contact point, etc? Or was it the double shot of vodka that I was served before the game?