From the Credit Union Times
In instances where money was fraudulently transferred out of the institution, 82% in the first half of 2012 involved wire transfers (with 14% ACH and 4% check writing and other). That compares to 2011 numbers of 91%, 9% and 0% in 2011, respectively
FS-ISAC said that several steps have proven effective in reducing account takeover fraud. It pointed to: Customer education; temporarily shutting down affected online customer’s access; Manual review of ACH/wire transactions above a certain dollar amount; and analysis of customer login characteristics/patterns.
Note the step "Manual review of ACH/wire transactions above a certain dollar amount". If you are thinking about making an ACH transfer of a large amount, you may want to check with your institution before attempting to transfer.