Table-topping Charlton suffered their first defeat of the season as they went down to a luckless defeat in their first-ever meeting with Stevenage.
The only goal of the game came from former Charlton midfielder Stacy Long's deflected shot in just the 11th minute.
Long cut in from the left to try his luck with a speculative 20-yarder, but it took a huge deflection to completely wrongfoot Addicks keeper Ben Hamer.
The League One leaders were unbeaten since losing at Walsall on 30 April.
Stevenage keeper Chris Day twice denied Charlton with excellent stops from Dale Stephens and Leon Cort, while Rhoys Wiggins hammered a shot inches wide.
Long almost doubled his tally when he saw a powerful drive beaten away by Hamer, while Jon Ashton came close to doubling the home side's advantage after the break.
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Stevenage manager Graham Westley told BBC Three Counties Radio:
"It's not been the best of times and we needed three points, there is no question about that. We knew we needed to step up our level.
"We have shown that we are capable of mixing it with the big boys when we get the levels right. When we play below our level we are capable of giving points away.
"There have been lessons learned and it is important for us to bed in this performance, remember what we did well and build upon it."
Charlton boss Chris Powell told BBC London 94.9:
"It's wasn't our afternoon but that happens in football. We've lost a game and our terrific unbeaten record ends.
"Now we move on and try to get another one.
"We need to regroup and get over the loss. We don't want to dwell on it."