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Voice recognition is always tricky as this poor Microsoft rep found out when he tried to show off Vista's voice recognition software.
The words that produced the sentence above were: Dear Mom (aunt) comma, fix aunt (let's set), delete that (nothing), delete that (so), delete select all (double the killer delete select all.
Perhaps Jobs' worst demo though was when the iPhone 4 demo went terribly wrong. First the Wi-Fi failed and then AT&T's 3G couldn't come through. Even after Jobs ordered the audience to turn off all their Wi-Fi devices, things still didn't go so well. Here's a hint: Never, ever rely on Wi-Fi for any tech. demo.