I once had an issue with a ticket purchasing website which accepted my "name+SERVICE" email address when I signed up, but it refused it when I tried to sign in

I once had an issue with a ticket purchasing website which accepted my "name+SERVICE" email address when I signed up, but it refused it when I tried to sign in later, or possibly just when I was trying to use the "forgot password" functionality - it's been a while. Either way, I was effectively locked out of a website that was storing my credit card information. Since I was in a hurry to buy the tickets (a last minute gift), I signed up for a second account without the "+SERVICE" section, bought the tickets, and sent them an email asking them to either merge the accounts, delete the old one, or otherwise allow me access the other account so I could purge my credit card data. What happened next was a bit shocking. They sent me an email back a few days later saying that they had fixed the issue with my account by removing the '+' symbol. The risk of exploit was relatively low due to the extreme obscurity, but it was technically possible for a short while for a person to go to my email provider and register the username "nameSERVICE" and access my account on the ticketing website, including my credit card information.