Guinea-Bissau came from behind to defeat Zambia 2-1 in an Africa Cup of Nations qualifier in Bissau on Sunday.
The loss leaves Sven Vandenbroeck's side facing an uphill struggle to reach next year's finals in Cameroon.
After four matches, they're bottom of Group K on four points, level with Mozambique and Namibia, who meet in Windhoek on Tuesday.
Guinea-Bissau, who qualified for the AFCON for the first time in 2017, currently top the pool on seven points and could seal qualification away in Namibia in November.
Justin Shonga opened the scoring for Chipolopolo, African champions in 2012, in the 13th minute, as Zambia looked like building on their 2-1 victory at home on Wednesday.
However, the hosts equalised eight minutes after the break when Stoppila Sunzu put the ball past his own keeper.
Eight minutes later, Zambia's troubles had been compounded, as Toni Silva nabbed a winner with an excellent header to beat Kennedy Mweene.
Shonga sailed a late effort over the box as the Chipolopolo's recent improvement under Vandenbroeck came to a screeching halt.