According to Wimbledon, the player challenge system works like this ..
Players have unlimited opportunity to challenge, but once three incorrect challenges are made in a set, they cannot challenge again until the next set. If the set goes to a tie break, players are given additional opportunities to challenge.
How the On-Court Challenge System Works
Each player/team is allowed a maximum of three incorrect challenges in a normal set after which they are not permitted to challenge again in that set.
If a set goes to a tiebreak, this limit of incorrect challenges is increased from three to four for the set.
Challenges may not be carried over from one set to another.
.. ..
Ok, enough. The above explanation is for non players. This blog is about how could a ball call in or out when only a fraction of the yellow-greenish furry thing bordering the line – the contact point of the ball is so small, yet it shows on the challenge screen un-proportionally large. I’m not good at science and unsure this would involve math or physics, but look at the shape of the dot on the review screen which represent the 
whole shape of the ball, which is approximately 6.7 cm (2.7 in.) in diameter .. .. my point is when it lands on the floor, only tiny portion would kiss the earth .. shadow shouldn’t count.
Oh well, just wondering.
Just saw another screw up of the umpire: Novak Djokovic served an ace to Roger Federer from ad court. The line judge called out, late. After challenging or reviewing, it showed the ball was in. Yet, Joker had to go for the second serve. Where’s justice?
