An early strike from Yannick Bolasie secured Crystal Palace a 1-0 victory over Derby, who missed a late penalty at Pride Park.
The pacey winger clinically scored the game's only goal for the promotion hopefuls, who looked good value for the three points until captain Mile Jedinak fouled substitute Ben Davies in the penalty area with three minutes remaining.
However, Sammon's powerful spot-kick was brilliantly deflected onto the bar by the Argentinian shot-stopper to consign Nigel Clough's side to a first home defeat of 2013.
The returning Wilfred Zaha had to wait just five minutes for his first opportunity - a long ball upfield was flicked on by Glenn Murray and Zaha's change of pace allowed him to sear in behind the Derby backline, only to then shoot straight at Adam Legzdins.
The breakthrough came in the 13th minute in freak circumstances. Eagles skipper Jedinak won a thunderous challenge inside his own half and the ball went flying up field to Bolasie, who took full advantage of Derby's AWOL backline to coolly round the keeper and fire home.
Palace might have had a second three minutes later when Jedinak's 25-yard thunderbolt cannoned off the crossbar, while at the other end only the excellent Damien Delaney's last-ditch challenge prevented Sammon going clean through on goal.
Derby began to play their way into the game; Jamie Ward had a volleyed effort parried behind by Speroni, before John Brayford's low cross was only half-cleared to Craig Bryson, who shot straight at the veteran keeper.
Sammon should have levelled seven minutes after the break when Chris Martin's clever overhead kick played Michael Jacobs in down the left channel and the substitute squared to the Irishman, only for Speroni to smother his effort after the former Wigan forward hesitated.
Legzdins did well to deny the visitors a second goal after 65 minutes when he brilliantly tipped Jonathan Williams' 20-yard effort over the bar, after the substitute had gone on a dazzling run from the halfway line.
Derby were enjoying the better of the second half, but when the ball dropped to Jacobs unmarked 12 yards out he wastefully dragged his volley wide.
The hosts were struggling to fashion clear-cut opportunities, but Speroni had to be alert to push away Richard Keogh's low drive from the edge of the box, before Sammon's penalty woe capped a miserable night for the home fans.
