By late afternoon, shares had plunged 8.2% to $4.47, negating the 8.5% gains achieved a day earlier.
On Wednesday, the fuel cell manufacturer soared on the announcement four buses had been deployed with Ballard fuel modules.
Scotland's Aberdeen City Council and Europe's fourth-largest bus manufacturer Van Hool NV launched the first four of a ten-bus fleet to be powered by Ballard technology, its 150-kilowatt FCvelocity-HD6 fuel cell.
"Arrival of the first buses signals a significant step forward in the Aberdeen Hydrogen Bus Project - the highest profile among a range of projects designed to create a hydrogen economy in the city and the ultimate fleet of ten buses will be Europe's largest hydrogen fuel cell bus fleet," Ballard said in a statement.
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