Do you mark your tennis balls over and above the number that's already stamped on them like golfers do? What mark do you put? Seems like everywhere I play people are using Wilson US Open, I'm going to start to put an additional mark on the balls.
Yes, more frequently where there are more than 4 courts together can become difficult to play and protect people from other courts from accidental new ball poaching .. I mark a big red florescent x on a side to help. More of a courtesy than paranoia.
To avoid losing tennis balls to adjacent court players, I tend to use less common types, like Dunlop Grand Prix, Wilson Australian Open (even though I'm in the US), etc.
But extra marks can’t hurt, just like cattle ranch branding in the old West. Maybe 777?
Can't say that I've ever been that worried about my ball being accidentally replaced by exactly the same ball. (Extremely rare disagreements are resolvable by whether one ball is cleaner or dirtier -- otherwise, what does it matter?)
Only rarely do I mark balls. (When I'm coaching and we open a new can for a Challenge Match, I'll mark 'em so they don't get mixed-in with the Hopper Balls.)
I just put a line on the seam with a Sharpie. After one match, I don't care that the mark has been obliterated. They're Hopper Balls after one match.
I don't care balls get mixed up at all. I'm not very picky about what I play with. The guys in my club league are always wasting time deciding whose ball is whose. I just want them to throw me one and get it over with.