Havant were left shattered after their 20-16 defeat against Sidcup at Hooks Lane in London one south.
The home side led 16-15 with just seconds left.
But they made a mess of a line-out five metres from their own line and paid a heavy price.
Sidcup won the ball from Havant’s throw and spun it wide to score a try in the far corner.
Even that was contested, with the Havant defence believing they had got the attacker’s foot into touch before the ball was grounded.
‘We are gutted,’ said Havant coach Will Knight.
‘The referee had already indicated that the next time the ball went dead it would be the end of the game.
‘All we had to do was catch the ball, clear it to touch and the four points were ours.
‘That is the second week running we have gifted the game to the opposition and it is frustrating.
‘We are good enough to be winning these games and that is what’s most disappointing.
‘The fire-power and capability is in the squad to see off these teams.’
The Kent visitors were quickest out of the blocks, taking the lead with a ninth-minute try before Joel Knight replied for Havant with a penalty.
Sidcup extended their lead with a penalty try after winger Loz Warren-West was adjudged to have interfered with a Sidcup attacker in a race for the loose ball over the line.
The home side trailed 12-3 at half-time.
But Warren-West made amends for his earlier indiscretion at the start of the second period with a well-worked score under the posts, converted by Knight.
Knight knocked over two more penalties to the visitors’ one, leaving Havant 16-15 ahead and on the brink of a vital win.
Then came the madness as Havant, instead of kicking the ball deep from a midfield scrummage, elected to run the ball.
When it was turned over, Sidcup kicked to the corner to set up the fateful line-out.
‘It’s another game that we have allowed to get away,’ said Knight.
‘We had done well getting ourselves back into the game and into a winning position.
‘It is our job as coaches to make sure that we don’t allow these defeats prey on the minds of our players.
‘We have to reflect on what went wrong.
‘But there were also a lot of positives to come out of the game.
‘There were a lot of good individual performances and our midfield did well again.
‘Our next league game is against the bottom club so we have to go there and make sure we win.’