Past experiments have shown that we tend to perceive moving objects as being slightly further along in their paths than they really are. This trait may have evolved to compensate for our sluggish visual system, which is always a split second behind reality.
In the tennis world, the illusion means line judges may call balls out when they were inside the line, according to researchers led by David Whitney of the University of California, Davis. They identified 83 erroneous calls by line judges in the 2007 Wimbledon tournament by analysing video of the matches. Of these, 70 were incorrectly judged out, while only 13 were incorrectly judged in - just the sort of bias to be expected if balls are perceived as having travelled further than they really have (Current Biology,DOI: 10.1016/j.cub.2008.08.021).
However, the bias may not be very strong, says George Mather of the University of Sussex, UK. He studied 1473 disputed points in footage of 15 tournaments and found a much smaller bias. He says the illusion distorts positions by just millimetres, a small effect given that tennis balls are more than 60 millimetres wide.