Two Robert Green errors and a stunning Matthew Taylor free-kick secured victory for Bolton at West Ham.
The Hammers keeper gifted Bolton their opener when he dropped a hopeful cross at Kevin Davies' feet and the striker poked the ball into an empty net.
Green then failed to hold the ball after saving Gretar Steinsson's volley and Gary Cahill was on hand to tap in.
Carlton Cole headed in from a corner to give West Ham hope but Taylor smashed in from 40 yards to secure the win.
Although Bolton's tactics were defensive at times it was probably no more than they deserved from a disciplined performance.
It might have been very different as West Ham's confidence was evident early on, as they looked to secure a third successive league win.
Their passing game quickly had Bolton on the back foot, with on-loan left-back Herita Ilunga a particular danger with some strong running.
Cole also looked a threat but failed to keep his shot down after a sharp turn.
However, it was Bolton who created the first real opening and Johan Elmander should have done better than to direct a tame shot straight at Green.
606: DEBATEBack came West Ham and Ilunga almost worked his way through on goal but was denied at the last by Jlloyd Samuel.
And as Bolton sat back, the hosts looked on the verge of making their dominance tell as shots came in from David di Michele and Valon Behrami.
But then in the space of four minutes, the game was turned on its head.
First, Green came out to collect a high ball but dropped it for Davies to gratefully slot home.
And then the unfortunate keeper failed to cling on to Steinsson's well-struck effort and Cahill was quickest to the loose ball to make it 2-0 to the visitors.
West Ham tried to push on and set about getting themselves back in the game, but their earlier swagger had gone and they looked stunned.
The hosts must have received a stern talking-to at half-time as they emerged after the interval with renewed fight.
And they almost halved their deficit when Cole's curler seemed to be heading for the top corner until Jussi Jaaskelainen clawed it away and on to the post.
Bolton hit back and Gavin McCann went close with a fierce strike before a Matthew Upson backpass nearly heaped more embarrassment on Green.
Then, from a set-piece, West Ham gave themselves a lifeline when Upson headed back a corner for Cole to nod in, with the help of a slight deflection.
West Ham were now in the ascendancy as Bolton sat back trying to protect their lead.
But despite having much more of the ball, the Hammers were finding it impossible to break their opponents down.
Instead it was Bolton who struck again when Taylor's brilliant swerving 40-yard set-piece easily beat Green and ensured Bolton left Upton Park with all three points.
West Ham manager Gianfranco Zola:
"It happens, sometimes it is the goalkeeper, sometimes it is the striker. I've missed chances from virtually the goal line.
"I'm sure Robert will come back and we'll be thanking his for saving a penalty or something.
"We have total faith in him and I have total faith in the whole team."
Bolton manager Gary Megson:
"We spoke about the way West Ham would come onto us and we had to try and negate that by taking the game to them and playing in their half.
"We've got work rate, resilience and character in spades but today we played some decent football.
"To come to a club that has won its first three games of the season here, to beat them on their home patch is a big thing for us."
West Ham: Green, Faubert, Neill, Upson, Ilunga, Behrami, Parker, Noble, Di Michele (Bellamy 54), Etherington (Sears 79), Cole.
Subs not used: Lastuvka, Lopez, Boa Morte, Mullins, Davenport.
Booked: Noble, Faubert, Bellamy, Parker.
Goals: Cole 69.
Bolton: Jaaskelainen, Steinsson, Andy O'Brien, Cahill, Samuel, Davies, Nolan, McCann, Muamba (JoeyO'Brien 80), Gardner, Elmander (Taylor 73).
Subs not used: Al Habsi, Hunt, Smolarek, Riga, Shittu.
Goals: Davies 30, Cahill 34, Taylor 86.
Booked: Nolan, Taylor, Andy O'Brien.
Att: 33,715
Ref: Mike Dean (Wirral).
BBC Sport Player Rater man of the match: Bolton's Gary Cahill on 7.09 (on 90 minutes).
Please note that you can still give the players marks out of 10 on BBC Sport's Player Rater after the match has finished.
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