Quote from lost100k:
I was doing average down many times, and never has problem until this time.
"Averaging down works really well until it doesn't. Then it REALLY doesn't."
Here is a scenario where averaging down is acceptable because you are planning your max risk in advance (though it's still not wise):
A stock sets up for a long or short, but it's not the highest probability setup (there another support or resistance level that may still be tested). But you are antsy to get into the trade because you might miss the move if you don't.
OK, if you're that impatient and afraid of missing the move, put on half your full position and place your stop just outside that next S/R level. IF it then moves against you and comes close to that next S/R level and you still feel the trade has merit, put on the rest of the position and keep your stop in place for the full position. Once S/R is broken all bets are off; honor your stop and re-evaluate the trade.
This method prevents you from taking a larger loss, and if the trade had gone in your favor right away, you would add the rest of your position to a winner.