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I don't think excuses the outburst, but it's well worth watching, the degree to which Serena Williams was royally screwed by bad calls in the 2004 US Open.
http://www.youtube.com/watch?v=xtkKdCeQgQM
Also, the Maria Jose Martinez Sanchez incident is, to me, pretty understandable, and the same sort of fucked-up-ness.
The best solution would have been for the judge to overrule the footfault call, then dock Serena a point for the outburst and leave the final point to be played. And frankly, though Clijsters had the edge in the match, many of the games were very long and closely fought, with both of them coming back from 15-40 to win ... so, yeah, anything could have happened, and it sucks tha Clijsters didn't take it all the way.
But yes, Williams behaved badly - understandable in the circumstances, but still over the top - and paid a fairly hefty price. With luck, this will spur the Open to extend the challenges they have for where the ball lands to something like a foot-fault, which would have settled the whole damn thing.