"Comes back & goes ''This isn't ours, it's a swap' so someone has literally left THEIR OWN BRA on a hanger in order to shoplift one AND I TRIED IT ON [sic]".
She continued: "F***in unbelievable imagine trying on a random persons bra actual kill me.[sic]"
Her story has now had over 128,000 likes and retweets on Twitter, and has seen many other people come forward to say that it happens all the time.
One tweeted: "I worked at Primark and this happens all the time, it's absolute horror. STOP DOING THIS PEOPLE."
"This reminds me of the time when I was shopping with my nan and I put her old shoes on the sale rack while she was trying stuff on," another posted.
"A lady put my Nan's shoes on and went to buy them and my nan had to tell her they were actually her old shoes."
A third said: "This used to happen a lot when I worked in a shoe shop, people walking out with our shoes and leaving theirs behind, or other customers trying on other people's shoes as they were trying on new ones."
It is not known which branch of the store the student visited.
A spokesperson for Primark told MailOnline in a statement: "We would like to apologise to the customer for this experience. We aim to provide our customers with an enjoyable shopping environment at all times and we are sorry that we didn't on this occasion."