1. Want to see what happens to irrational markets? In 1999-2000, the NDX went a little more than double over the span of 5 months. When it turned, 25% was lost in 7 trading days. After a strong bounce, it dropped again losing overall 35% in 3 weeks. 
2. After another longer bounce and drop, the total was 40% down. It managed to recover ~62% of that in the months that followed.
3. BTFD crowd rejoiced that we are off again - not knowing that 2 years later the index would lose 85% of it's value.
4. At almost 12k today, the NDX would lose 3000 points in 7 trading days if it did the same and 4800 points in 2 months. Overall, the comparable loss would target 1800 on the index.
5. Crazy right? It happened once and that was when we had a GOOD economy. Now, tens of millions unemployed, economy deteriorating, and companies valued at hundreds of billions with little sales to justify. Different this time? I don't think so.
6. 2007 - Not quite as dramatic runup, but the turn dropped this 12% in a week. 25% in just over 2 months. Overall...65% down.
7. You think 2000 was a one-off? From ~Mar 1928 to Aug 1929, the DOW nearly doubled. The turn was not quite as fast but in just over 2 months, it lost 40% on the first phase. A few years later, it would lose 90% of it's value. Imagine the DOW dropping to under 3000 from now.
