I tell them every single time. League, for fun, and definitely in tournaments. Karma is a b1tch and she'll come back around and bite you in the ass.
I don't play much 8-ball, but you'd be surprised how often this comes up in 9-ball. I don't think shooting the wrong ball means you aren't paying attention. I've done this before playing on a 9 foot table with a light meant for an 8 foot table and concentrating so hard on the match, that I somehow overlooked the lonely 6 ball on the other end of the table hiding in the shadows. I don't have the best eyesight in the world, either. I was so caught up at how I was going to run out the 7,8,9 that I didn't even see the 6. Once I was told there was still a 6 on the table, I was way out of line and still ended up losing the game. I also felt very embarrassed, too.
This happens and I tell my opponents every single time because I wouldn't feel good about myself winning a game that way regardless if it was for fun or money.