Leads in a GoToWebinar event program have a "registration error" status.
If this is affecting all new leads registering for the webinar: This is caused by Marketo losing connection with the GoToWebinar service. To resolve this
Go to Admin > Launchpoint.
Open the GoToWebinar service.
Re-enter your GoToWebinar credentials.
Once this is done, you can go back to the webinar program and change the status of the affected leads from "Registration Error" to "Registered."
If this is affecting only specific leads registering for the webinar: Check the name, email, company, and job title fields for any special characters. GoToWebinar often doesn't allow special characters and will throw an error.