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.
The sites that let you sign up using the + and then don't work later tend to be using URL encoding/decoding and converting that + into a space in some cases but not in others.
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.