Which mistake was that?
The whole Iraqi 'WMD' debacle.
Powell has done very well. Others like to forget, but I remember a few years back when Iraq turned very bad as the insurgents implemented some new tactics and spiked US casualties one particular month.
Powell crawled out of a hospital bed where he'd had surgery for cancer and answered reporter questions on camera just outside the hospital door. He defended GW Bush and the administration effort in Iraq. He did this while our conservative senators were cowering under a table all week long. Nary a one was to be found in front of a microphone defending the administration.
Powell did what the rest of the entire GOP would not do that day. Performance above and beyond the call of duty.