Space Shuttle launch ever scrubbed?

many times. the t - 9 minute mark is the last chance that controllers can call a no go if they see something that's not quite right in the systems they monitor or the weather. the most common cause for a scrub at this point is due to weather.

after t - 31 seconds, the shuttle's computers take over all the vehicles critical functions and at that point, the shuttle's computers can call an "RSLS" abort (Redundant Set Launch Sequencer). which has happened 4 times in the shuttle program history.

Many. There had been quite a few "Redundant Set Launch Sequencer"(RSLS) aborts, that even stopped the launch just milliseconds from igniting the solid rocket motors. "Redundant Set" in the term means the computers in the space shuttle, which are all operating the same software in a synchronized way.

As example, here is a youtube video of one, showing how water jets are directed at the end of the Shuttle after the engines stopped for fighting possible fires.

http://www.youtube.com/watch?v=3VELYTzUrjc