Alexis Sanchez benefitted from Adam Federici's error to send Arsenal into a record 19th FA Cup final after an extra-time victory over Reading at Wembley.
Federici allowed Sanchez's shot in stoppage time at the end of the first period of extra time to go through him at the near post to hand the holders the most fortunate of victories.
Sanchez, a £35m addition from Barcelona in the summer, had earlier given the Gunners a first half lead after being played in by Mesut Ozil but Gareth McCleary's equaliser 10 minutes after the break had Reading dreaming of a maiden final appearance again.
Substitute Gabriel and Aaron Ramsey both hit the woodwork in a frenetic end to normal time but as the nerves started to show in extra time Federici scrummed to the pressure to send holders Arsenal into their sixth final under Arsene Wenger.
Steve Clarke's side more than played their part and despite eventually losing out to their Premier League opposition there is reason for optimism that their season near the lower echelons of the Championship will not be repeated next term.
With a 100% losing record in 12 meetings against Arsenal, Reading were seeking to make it 13th time lucky on their first FA Cup last four appearance since 1927 and put behind them a harrowing domestic campaign.
Manager Clarke, a winner of the competition in 1997 with Chelsea, was boosted by the return of Pavel Pogrebynak from a calf problem in one of two alterations from the mid-week loss to AFC Bournemouth.
Gunners boss Arsene Wenger meanwhile could call upon a squad which was uncharacteristically free of serious injury and led to the recall of the fit again Mathieu Debuchy while Danny Welbeck replaced Olivier Giroud.
Having won their last eight Premier League games in succession, the North London club were huge favourites to reach a second final in as many seasons but memories of their struggle against Championship opposition in Wigan Athletic in last season's semi-final would be a source of optimism for the Royals.
Any nerves either side might have had were none existent in the early moments as chances came at both ends. First, Federici got down low to keep out Per Mertesacker's header before on the break from the subsequent corner Jordan Obita stunned the hands of Wojciech Szczesny on his first appearance since March.
Chelsea loanee Nathaniel Chalobah's volley was then deflected over the Arsenal bar in another opportunity for the underdogs as the holders, despite Ozil and Santi Cazorla's off-target efforts, began slowly.
The 11-time winners' sluggishness continued for the remainder of the first half with Reading, who are hovering above the relegation zone in the second tier, performed with the intensity of a team who had waited nine decades for a semi-final appearance in the world's oldest cup competition.
Reading's appetite was giving the holders' little room for manoeuvre but the moment their pressing game relented, Arsenal found the break-through. Ozil played a delicious curled pass into the path of Sanchez, who controlled, turned past McCleary and tucked past Federici.
However, any fears the opening goal would open the floodgates were extinguished 10 minutes into the second half when Reading grabbed an unlikely leveller. Pogrebnyak surged to the byline and though McCleary turned the cross goalwards with an outstretched foot it took a deflection off Kieran Gibbs and a fumble from Szczesny to see the ball bounce over the line.
With their top flight opposition rattled, the reinvigorated Berkshire-based side set about surging into the lead but Daniel Williams volleyed wide from the edge of the box after Mackie's cut-back.
The first sustained period of pressure from Arsenal in the second half did yield a chance to retake the lead as substitute Gabriel, who had replaced captain Per Mertesacker, forced Federici to palm the ball onto the crossbar from a fine flicked header.
The Brazilian January signing was then guilty of another miss from point-blank range from Ozil's free-kick, heading over from six yards out amid another defensive lapse from Reading.
Sensing an unlikely victory, Chalobah almost caught out Szczesny with a dipping shot which the Pole nervously punched behind, but from the resulting set-piece Arsenal should have retaken the lead but Ramsey inexplicably hit the post from two yards out after beating Federici.
Pogrebnyak could have won it for Clarke's side but spurned a two-on-one situation after escaping the attention of Gabriel, before Ramsey's radar was again askew as he poked over Gibbs' cross.
But as neither side took the opportunity to grab a late winner the game slipped into extra time where the nerves of supporters, not least the fatigue of both teams would be heavily tested.
Following an entertaining end to normal time, the extra-time period saw the game become a predictably scrappy contest, where only a moment of brilliance or a mistake would decide who would return to Wembley in May.
And that ignominy would befall Reading's Federici who allowed Sanchez's shot in stoppage time at the end of the first period of extra time to squirm under his body and over the line.
Reading pressed for an equaliser in the closing minutes, but as the chances continues to flow at the other end as Giroud stuck the outside of the post, the North Londoners held out to set up another final against either Liverpool or Aston Villa next month.