Why do event registration visitors receive a "UNKNOWN_EXCEPTION" error on the thank you page after a registration?

When a visitor completes their registration for your event, they may receive an error that looks like this:

Salesforce Error: UNKNOWN_EXCEPTION: An unexpected error occurred. Please include this ErrorId if you contact support: 90008820-00000 (152253525)

This error means that only Salesforce has access to the debug log for this particular error. In order for us to assist you on an error like this, please do the following steps.

1) open a support ticket with Salesforce with the specific error code provided, asking them what would be causing the error

2) after receiving the response from the Salesforce support team, open a support ticket in our support portal with the information from Salesforce

3) our support team works with you through the challenge