The Rocks' lead at the top of the Ryman premier is down to a single point after they conceded an injury-time winner to lose at home to bottom side Grays.
On an afternoon when they were way below their best, Bognor looked like they had salvaged a point thanks to an 85th-minute equaliser by James Fraser.
But Grays, who had taken an early lead through Kieran Bishop, clinched the win in the final minute of five added on when Lheureux Menga headed in a cross to leave the visitors jubilant and the hosts and their fans feeling very flat.
The Rocks, who had Harvey Whyte sent off late on for two bookings, will have to lift themselves quickly to get the promotion bid back on track when they go to Burgess Hill on Tuesday night. They will go into that game with a lead of just a point over Needham Market, with third-placed Havant another point behind in third spot in what is becoming a closely-fought top group.
It was top v bottom at Nyewood Lane although the Bognor management were quick to insist before the game they'd be treating it like first v second.
There was the welcome return to the starting line-up of midfield maestro Dan Beck for his first start after a long spell out injured.
A fifth-minute Beck corner was met by Sami El-Abd's goalward downward header but it was hacked away by a defender - before Elijah Adebayo, last week's hat-trick hero, beat keeper Amadou Tangara to a cross but took the ball out for a goal kick.
Bishop gave the Rocks an early shock by firing Grays into the lead when given far too much space in the box to collect a ball in from the right, turn and shoot low past Dan Lincoln.
Bishop was almost on target again when he closed down Lincoln after a back pass and blocked the keeper's clearance, but it flew out for a goal-kick.
It was a lethargic first 20 minutes from the Rocks as they failed to respond to their early setback with any decent chances of their own.
Calvin Davies woke the crowd up with a low cross that fizzed across the face of goal but Adebayo, though closest to it, couldn't connect.
Bognor were forcing a series of corners but that was as threatening as they got during the first half-hour.
The Rocks had a chance to put Grays under pressure on 36 minutes when Alex Parsons was fouled on a run down the right, earning Joe Payne a yellow card. James Crane headed on Beck's free-kick but Chad Field sidefooted wide at the far post.
One promising Rocks attack led by Adebayo ended in Crane finding Parsons with a far-post cross but the offside flag was up.
The Rocks were straight on to the attack in the second half, no doubt after some strong words from the management at the break, but a corner was - following the pattern of the first period - cleared easily.
Kwabena Osie was booked for chopping down Fraser six minutes into the second half and in the break in play, Whyte replaced Davies.
Matthew Baxter sent a long-range drive wide of Lincoln's right-hand post after a quick break by Grays. Then the Rocks were straight up the other end and Adebayo's low shot was held at the second attempt by Tangara.
Lartey Sarpong was next to test Lincoln, shooting into the side netting from an angle. At the other end Parsons curled a cross on to the roof of the net.
Ollie Pearce replaced Beck just before the hour, while Grays brought on Barney Williams for Harry Watkins.
Osie shot high and wide after Joao Carlos's trickery set him for a 25-yard tilt at goal but Grays were giving as good as they were getting in the second half.
Whyte picked up Bognor's first booking of the afternoon on 66 minutes for hauling down Carlos. A second Rocks change on 69 minutes saw Gary Charman replace Tuck.
The Rocks finally managed to go close to an equaliser on 71 minutes when first Charman had a header blocked from a corner then Muitt sent a volley towards the top corner only for it to be deflected over by a defender's head.
Crane hooked a 12-yard shot just wide as the Rocks continued to pile bodies into the box with 13 minutes left.
Grays brought off Bishop for Menga. Carlos went into the book for time-wasting with ten minutes left, with the Grays bench complaining at what they thought was a shocking decision.
Still more corners came for the Rocks but Tangara and his defence coped with them.
El-Abd was booked for protesting too vociferously that he should have had a penalty when he won only a corner.
But the series of corners finally finally told as Grays only half-cleared one and when the ball came back in Fraser rose to head in from close range.
Fraser was then named man of the match just as he curled a dangerous free-kick into the box that just eluded everyone.
The Rocks looked like they thought they had a winner in them and had an El-Abd header cleared off the line as they forced some strong late pressure.
There was a final twist in the tale but it was at the wrong end as far as Bognor were concerned.
Well into five minutes of injury time Grays had one last attack and when it brokw down and Adebayo tried to launch one last Rocks break, he was robbed when he perhaps should have made sure of a clearance - and when the cross came in from Sarpong on the right, sub Menga glanced a header past Lincoln at his near post.
Grays celebrated the win with their travelling fans at the end as the Rocks trooped off reflecting on a day to forget and a big blow to their promotion hopes.
Rocks: Lincoln, Davies, Crane, Tuck, El-Abd, Field, Fraser, Beck, Adebayo, Muitt, Parsons. Subs- Charman, Budd, Dolcetti, Pearce, Whyte.
Grays: Tangara, Mahal, Payne, Sarpong, Campbell, Ashton, Watkins, Osie, Baxter, Bishop, Carlos. Subs - Williams, Kucuk, Williams-Agunabor, Mukanya-Kabobola, Menga.
Ref: Jacob Miles
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