"We started off terrifically then we started showboating and got carried away," declared a numb Alan Pardew.
"Without a shadow of a doubt this has ended up a most frustrating day of my managerial career."
Following five successive draws, the Hammers took control in the opening 18 minutes and looked destined for all three points rather than the habitual one.
Indeed, with just 41 seconds on the clock Jermain Defoe had carved down the left flank and squeezed a tightly angled shot between Russell Hoult and his near post to claim his 11th goal of the season.
Not to be outdone, strike partner Brian Deane soon joined him on the scoresheet when he sidefooted home from Don Hutchison's return pass after ten minutes.
Debutant striker Deane then put West Ham into an all but unassailable lead when he nodded the alert Defoe's quick free-kick past Hoult.
"I have never seen a West Brom team start so pathetically," admitted a gobsmacked Gary Megson.
"We gave West Ham a three-goal start."
On 25 minutes Rob Hulse looked to have grabbed a mere consolation when he nipped in between Christian Dailly and David James to nod home from 12 yards.
Six minutes before the break Hulse then gave Albion real hope when he outmuscled Daily and curled home a superb 20-yarder.
And an interesting second half was assured when Defoe, who had already harshly been booked for diving, recklessly scythed through Sean Gregan to collect his second red card of the campaign.
While Albion emerged with renewed purpose for the second period, the ten-man Hammers were instantly on the back foot.
The smart money was on a Baggies leveller, but few would have bet on Deane completing a bizarre and unwelcome hat-trick when he volleyed a visitors corner beyond the bemused James to make it 3-3.
With Albion totally in the ascendancy there was only ever gong to be one outcome and sure enough 15 minutes from time substitute Lee Hughes capitalised on James' ineffective punch to clinch victory with a 15-yarder that deflected in off Tomas Tomas Repka.
"I shall not forget this match in a hurry and I won't be letting the players forget it either," added a shell-shocked Pardew.