Why would someone complain about this?

I don't have any information about the type of device, alas. But I am very curious about what the registrant described below could have been doing. We had a webinar set up with registration that sends the confirmation and reminder emails. In those emails we instruct the registrant to click/tap the big, blue "Join Webinar" button to attend the webinar.

The person who was signed up complained:

"The code didn’t work for [the webinar]. It was in a loop sending me to register. I was already registered that’s how I got the code I hope you get the problem fixed before [the next webinar]."

What could this person have possibly have been doing? I guess I need even more instructions for people who need additional help.