I'm bad at line calls if they are really close. I'll see the ball out if it clips the outside of the line but I'll correct the call if someone tells me I'm wrong (happened quite a few times). Also, if it's near me, I tend to call it out accidentally, and correct myself. But I really hate when my partner (in doubles) makes horrid calls. If he says a serve (I'm very generous with those) is out when it obviously hit hit the line, even if it would be an ace, I'll correct him and give it to my opponent.
If my opponent cheats, I'll let him know I'm on to him, and give him hell till he stops.