Really good reasons to give line notes whenever an actor messes up a line . . .
1) It starts the stage manager noting an actor relationship.
2) Even if you think it was one time mistake, the actor may be thinking that was the one time they said the line right.
3) If an actor, who previously got the line right, gets it wrong, by giving him the line note might remind him to give you a note . . . "Oh, yeah, I messed up the line because . . . "
a) I was totally in the wrong blocking, can you remind me of my blocking.
b) So-and-so gave me the wrong line, we changed that line.
c) The line is correct, the director said I could change. (the playwright said I could change it . . . etc).
d) I went up because my prop wasn't where it was supposed to be . . .
e) I missed my cue because I realized I was wearing the wrong shoes.
80% of the time, I guess, a line note is simply about the actor not knowing the line, but if the actor drops a line, there might be a very good reason - and it might not be the actor's issue with the line.