Jamie Allen’s late goal saw Burton reach the fourth round of the League Cup for the first time in their history as the League One side came from behind to defeat Burnley 2-1 at the Pirelli Stadium.
Kevin Long headed the Premier League strugglers in front just before half time but Liam Boyce equalised and then set up the winning goal for Allen, when Burnley were piling on the pressure.
Jake Hesketh should have made the winning margin more comfortable but saw his close-range shot strike the post in injury time.
Steven Defour made his first appearance since January after recovering from knee surgery as one of six changes made by Burnley boss Sean Dyche. Tom Heaton, Long, Stephen Ward, Dwight McNeil and Chris Wood were also brought into the starting line up.
There was little between the two sides during the first half. Burton twice threatened as the half-hour mark approached as a dangerous David Templeton cross just eluded Ben Turner, before Scott Fraser should have done better than steer his shot straight at Heaton.
Burnley twice had the ball in the net through Wood and Long, but the offside flag was raised on each occasion.
Wood flicked an effort narrowly wide before the Premier League team went in front after 40 minutes when Long headed in a Gudmundsson corner. Dimitar Evtimov got a hand on the ball but could not keep it out.
Evtimov did well to repel a powerful Gudmundsson effort from the edge of the box before a swift counter-attack from Burton resulted in Boyce scoring the equaliser, sliding the ball beneath Heaton.
Substitute Ashley Barnes had barely been on the pitch a minute when he crashed a shot off the foot of a post. Ashley Westwood wasted a chance and Wood was denied by Evtimov as Burnley looked to turn the screw.
However, it was Burton who struck the decisive blow with seven minutes left as Boyce slipped a perfectly-weighted pass through to Allen inside the penalty area and he fired past Heaton.
Hesketh somehow managed to send his shot against the post from three yards out as he slid in the convert a driven cross from Marvin Sordell but it mattered not and Burton progressed.