Shares in online retail giant opened 1.5 per cent lower at 3,630p after a day-long outage prevented thousands of customers from taking advantage of a 20 per cent discount.
Banners on the website advertised a 20 per cent discount on "Every. Single. Thing", causing customers to flock to the site. But it failed to stand up to demand, with the mobile and tablet sites collapsing altogether, and desktop users prevented from paying for their items.
Asos advised frustrated customers to keep trying.
One customer said the outage "feels like Black Friday".
Customers' frustrations were compounded when Asos said the discount would not be extended into today.
This morning the desktop site appeared to be fully operational, although the mobile and tablet sites were still down.
Last year was a rocky one for the retail giant, which was hit beset by the twin challenges of weakness in its international trading and a warehouse fire, which cost it £30m.
In April this year it posted sales up 14 per cent, although profit before tax was down 10 per cent.
"Momentum in the business is building," said chief executive Nick Robertson.
Shares later rebounded, trading 0.49 per cent up in the early morning.
Asos has been contacted for comment.