Somerfield has launched an inquiry after video clips of people performing stunts wearing the firm's uniform were posted on the internet.
Skiing with a floor cleaner was among the pranks.
The clips, on the YouTube website, allegedly feature staff of the supermarket chain "skiving" from work.
The antics include a man balancing on a beam in a store room, another riding on a floor cleaner and somebody racing round a car park on a pallet lifter.
Somerfield said it was "looking into the incidents".
The clip was posted after one Somerfield worker wrote on the video-sharing site: "I declare the underground YouTube Somerfield competition open to all Somerfield staff", trade magazine The Grocer reported.
The aim of the contest was to "give all senior management the finger", the message said.
In one clip, a youth appears to hit his head on an air-conditioner unit after "skiing" on a box, being pulled along by an electric floor cleaner.
Another, showing a worker dancing, is labelled: "We Somerfield lot really do get paid to do nowt."
And a video called "Somerfield massive" shows an alleged employee walking along a beam, while another shows a worker racing around a car park on a pallet lifter.
Another clip is a spoof advert for the chain.
It features a man dressed in Somerfield uniform, ignoring somebody trapped under a trolley, swearing at customers and pushing them into a display pile of kitchen roll.
A Somerfield spokeswoman said "We are looking into the incidents and will take any necessary action where appropriate."
YouTube, which Google bought in November last year for $1.65bn, claims clips on its site are viewed 100 million times every day.