There is a site I use that does this. I try and use $sitename@domain.tld as the email for most things I sign up to. Occasionally this gets wonky like the site in question because it was originally a Patreon sub for access so the email for the site was patreon@domain.tld then they switched to their own payment system but my email for them is now locked to patreon@domain.tld.
So when I log in with sitename@domain.tld it thinks it's new account and just signs you up as a new user rather than saying you don't have an account.
Now imagine having a bad day and you try and sign in with 5 different emails because you forgot wha one it was and snow you have 5 different "new account" flows with all the bullshit +12+24+48 hour onboarding/retention/marketing emails for all 5 email addresses.
My fault entirely obviously but it is a little mad.
So when I log in with sitename@domain.tld it thinks it's new account and just signs you up as a new user rather than saying you don't have an account.
Now imagine having a bad day and you try and sign in with 5 different emails because you forgot wha one it was and snow you have 5 different "new account" flows with all the bullshit +12+24+48 hour onboarding/retention/marketing emails for all 5 email addresses.
My fault entirely obviously but it is a little mad.