A drunken reveller launched an unprovoked attack on a man at Melksham's Party in the Park this year.
Leslie Gregory, 52, used the beer bottle that was in his hand to smash it into the head of his victim, who had gone for a day out with his family.
But after hearing he should not have been drinking on his medication and suffered a broken leg when he fell over after the attack, a judge imposed a community order.
Hannah Squire, prosecuting, told Swindon Crown Court on Friday how victim Andrew Kopec had gone to the festival on Saturday, July 13.
A man nearby who was swaying and staggering steadied himself on Mr Kopec, and he asked him what he was doing.
Gregory made no reply, but punched Mr Kopec to the shoulder and struck him across the forehead with the brown beer bottle.
A witness told how he saw the defendant pull his victim from his chair and had him in a head lock as he struck him with the bottle.
The victim swung a punch in self-defence, which missed as he stumbled backwards, and Gregory fell over, badly breaking his lower leg.
Mr Kopec, who had blood streaming down his face, was taken to hospital where steristrips were applied to two wounds on his forehead.
Gregory, of Catalina Court, Bowerhill, Melksham, pleaded guilty to actual bodily harm.
Alex Daymond, defending, said his client was diagnosed with diabetes in 2007 which meant he had to reduce his alcohol consumption, and was also on antidepressants following the death of his mother towards the end of last year.
Judge Tim Mousley QC imposed a one-year community order with an alcohol treatment programme, 150 hours of unpaid work and pay £500 compensation.
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