"I look at her, she's looking at me, I pick up the mayo, I'm waiting for her to be like haha jk.
"Nothing," he says.
"Me: 'You know mayonnaise has eggs in it right?'
"THIS. GIRL. JUST. STOOD. THERE. SILENT.
"She stood there for a second.
"V: 'N-no it doesn't, I get mayo every time, are you sure?'
"Me: 'Yes ma'am, mayo has egg whites in it.'
"I felt so horrible, she stood there with such a distraught and defeated face, I had shattered this poor girl's world."
The Subway employee says by then a couple of other people were waiting in line, so he had to finish the girl's order and move on.
"Me: 'Would you like the mayo on it ma'am?'
"V: 'Sure, go ahead.'
"She sounded so done, so defeated."
He gave her the sandwich, with the very non-vegan mayo, she paid and left.
When he wished her a nice day she just said "yeah, I'll try".
The post has been shared nearly 44,000 times and received more than 66,000 likes on Facebook.
A lot of people had similar tales to share.
"Something similar happened to my sister's friend when she said the only thing that gets her through being vegan is smothering everything in gravy. my sister said most gravy isn't vegan. She was absolutely devastated," a Facebook user commented.
He also posted it to Reddit where it received more than 2000 comments.