The reserves were left cursing their luck on Monday evening as a goal four minutes into stoppage time gave Charlton Athletic a 1-0 victory at The Valley.
The young Hammers side looked to be holding on for a hard-fought point against their more experienced rivals, only for striker Alex Varney to head a late, late winner.
Following Friday's 2-0 defeat at Ipswich Town, Kevin Keen again named a young team, who battled bravely against an Addicks side coached by former Hammer Mark Robson.
Not surprisingly, the hosts enjoyed the greater share of possession and opportunities in front of goal, and Hammers had goalkeeper Stephen Bywater to thank for keeping the scores level.
The 24-year-old produced two stunning saves in the space of one first-half minute to deny Jeffers, and made a superb block to thwart Varney after the interval.
The visitors' first real chance of the night arrived on 55 minutes, when Kyel Reid's firm header was kept out by Danish goalkeeper Stephan Andersen, while defender Jon Fortune made an excellent last-ditch tackle to prevent Hogan Ephraim from breaking through.
With 10 minutes remaining, it looked as though Hammers would gain the upper hand for the first time in the match, when Charlton had midfielder Lloyd Sam sent off for a wild lunge at substitute Jack Jeffery, but despite the one-man advantage, they couldn't find a breakthrough in the closing stages.
Then, with just seconds remaining in stoppage time at the end of 90 minutes, the Addicks grabbed their winner to leave Hammers heartbroken, as Youga crossed from the right for Varney to head home a simple chance.
Hammers: Bywater; Andrews (Ngala 86), White, Henry, Widdowson; Reid, Collison, Baradji, Welsh (Jeffery 71); Hines, Ephraim. Subs unused: Hales, Blackmore, Fraye.