As an aside, if you do something to Olympic standard and practice it day in day out, wouldn't having a missing hurdle look slightly odd to you? There'd be larger gaps between them or a huge gap at some point, I'd have thought that someone would have noticed it.
kelly sotherton tweeted something about it not feeling right and thinking there were only 9 hurdles. that's the only reason anyone knew what had happened
if they'd spaced them wrongly everyone would've noticed as their stride pattern would've been out but because they missed number 10 out it just left them with a longer run off which is much less likely to be noticed, it's just head down and sprint til you hit the line at that point