I'm having an issue on a site running EE v2.5.5 and Cartthrob 2.3.6. The vast majority of users are able to register no problem. However, some users intermittently go through the registration process, receive a confirmation email, but when they try to login they get the error: The existing username and/or password you submitted are not valid.
When you check in the backend of the site the member is not registered. I can't for the life of me find any pattern. I thought it might be something to do with people registering from work, where companies have some kind of excessive caching on their servers which is confusing EE somehow.
If anyone has experienced something similar please feel free to share the pain, and even better a solution!