A Mark Noble penalty gave West Ham an away goal in their Europa League play-off first leg, but Denis Alibec rescued a 1-1 draw for Astra.
The Hammers, bidding to reach the group stages for the first time, looked set to take a lead back to the London Stadium next week.
However Romanian champions Astra Giurgiu, who beat Slaven Bilic’s side in last season’s qualifiers, stunned the visitors again when sub Alibec hit a late equaliser.
Boss Bilic fielded a team of youngsters for last year’s clash but is taking this season’s competition seriously and brought a strong squad, even though he has a growing injury list.
Already without Dimitri Payet, Manuel Lanzini, Sofiane Feghouli and Aaron Cresswell, Bilic had 24 hours earlier been rocked by the news that his £20million record signing Andre Ayew would miss four months with a thigh injury.
However, he was able to hand a debut to another new face, and Argentine striker Jonathan Calleri almost got off to a dream start with a header from Noble’s cross which was palmed out by Astra keeper Silviu Lung.
Backed by 400 travelling fans in the 8,500-capacity Marin Anastasovici Stadium, West Ham shrugged off the evening heat to press for a quick opener and perhaps settle any lingering nerves from last year’s embarrassment.
Michail Antonio, now playing on the left wing after a poor display at right-back against Chelsea on Monday, had a header cleared off the line by veteran midfielder Filipe Teixeira, formerly of West Brom and Barnsley.
The lively Calleri thought he had broken the deadlock when he powerfully nodded Tore’s corner goalwards only to see it somehow kept out by by a fine save by Lung.
But a minute before half-time the visitors were gifted the lead when Cristian Sapunaru handled in the area and skipper Noble stepped up to send Lung the wrong way with the spot-kick.
Astra boss Marius Sumudica had made a surprise appearance on the bench having been serving a lengthy ban for betting on matches, and he must have had some choice words for his side at the break because they looked a lot sharper in the second half.
Yet they did not look capable of grabbing an equaliser, while Antonio should have doubled West Ham’s lead with 10 minutes left after a strong run from youngster Marcus Browne only to fire into the sidenetting.
Ninety seconds later the visitors were made to pay as they failed to clear a near-post corner and Alibec swivelled to rifle a fierce drive across Darren Randolph and into the top corner.