Mine as usually gluten related, as I'm coeliac. I was on holiday in Athens with a couple of my mates who grew up there. I was really enjoying the local food there but we decided one day to give this pizza place a go because they offered gluten free. Cool! We ordered and checked in English and Greek that it was GF. I demolished my pizza and it was delicious. Got to a couple hours later and I was throwing up so bad. That sort where you physically can't stop it. We were at somewhere grabbing a drink at the time, so I was curled up around their loo, throwing up. We finally made it back to their place after more puking. Then the other digestive issues kicked in. Anyway, I left a review of the place saying it's genuinely nice food but don't go there if you're coeliac. They were pissed and said I needed to take my review down. Messages flying back and forth. They offered me a free pizza. Bro, no. I don't wanna die. They said that those pizzas had "lower levels of gluten". That's not a thing. It's gluten free or it isn't. They told me they're made with spelt flour. Not GF in the slightest. They said they'd try and make a GF pizza (wouldn't trust it even if they did). They emailed back shortly after saying it was too difficult. Safe to say, if I ever go back to Athens, I'm never eating there again. I feel bad for the server because he would've only known what they told him.
I've had a few run-ins with places and me being coeliac (diagnosed as a toddler). But this one stuck out to me as particularly bad.