Step 1: Have USD available for spending on mtgox.com.
Step 2: Put in a buy order large enough to drain your account. Low enough under the current trading price that it will not execute immediately.
Step 3: Withdraw all USD funds.
Step 4: Wait for market to fall enough to meet your order.
Step 5: ...(self explanatory)...

There's a bit of luck in being able to take advantage, obviously.

I would suggest you take the site down asap until this is corrected or publicly show how this order will never execute:

==========
Welcome 0.00000000 ฿TC 424.44901
Buying  138468.901  0.01  Active  1384.69  06/26 15:27  cancel
==========

I cannot guarantee this order will execute but from everything I've observed about the new trade matching code I have no reason to believe it will not.

At the very least this could be used to influence market conditions if it is only a display bug.

bitcoin-dev: http://sourceforge.net/mailarchive/forum.php?thread_name=C9421AA2-D741-4989-9DA8-395D1F532F52%40jrbobdobbs.org&forum_name=bitcoin-development
f-d: http://lists.grok.org.uk/pipermail/full-disclosure/2011-June/081682.html