Jet Streaming came with a wet sail to grab the Win Big With Tote Handicap at Galway in the dying strides.
Having headed the front-running Stormy Belle, So You Thought looked like giving Dermot Weld his first winner of the 2017 Festival.
However, he was always close to the pace and his effort began to take its toll as the post loomed, with Danny Sheehy bringing Jet Streaming (7-1) with her challenge down the outside to get the verdict by half a length.
"Danny is a young lad with loads of experience and he was unfortunate to get a bad fall during the winter," said trainer Adrian Keatley.
"It just took him half a dozen rides or so to get back into the swing of things but he's strong.
"I'm delighted the filly won and she'll definitely step up in trip. With another furlong she'd have won a bit easier and she'll be even better next year.
"All that happened to her last time was that she got caught out for a bit of toe for 50 yards or so."
Sheehy said: "I was a bit slow away but it kind of suited as I was told to take my time.
"I got a lovely run up the inner and when the gap came I had to switch.
"There's no doubt she'll stay further."
Weld's Sansibar Jewel was all the rage for TheTote.com Maiden but faded tamely as the Michael O'Callaghan-trained Perfect Soldier (5-1) just held the late thrust of newcomer Giorni Felice.
O'Callaghan said: "This is unbelievable - a dream (first winner for Michael O'Callaghan Racing Club). I thought Delegating (third on Monday) might do it for us the other night and I've never shouted a winner home more.
"This horse always hits a flat spot and I told Sean (Davis) to give him a few back-handers in the middle of the race if he needed it. It was a savage ride.
"I'm more delighted for the young lad on his back than anything else. He's been with me a long time and he's not from a racing background."
Magna Cartor sprung a 20-1 surprise in the Win Big With Tote Handicap.
Shark Hanlon's charge set sail for home early enough in the hands of Rory Cleary but held on well from Water Sprite.