Error: The name can only contain underscores and alphanumeric characters. It must begin with a letter and be unique, and must not include spaces, end with an underscore, or contain two consecutive underscores.

I recently encountered this error, when testing my visualforce page preview by passing in id parameter in the url.

The error was caused by the URL

The URL that generated the error:

https://<yourDomain>/apex/<yourVisualforcePageName>&id=<yourRecordIdOfStandardController>

This is the correct URL:

https://<yourDomain>/apex/<yourVisualforcePageName>?core.apexpages.request.devconsole=1&id=<yourRecordIdOfStandardController>

The difference between the 2 is that, after the <yourVisualforcePageName>, we must add the 'core.apexpages.request.devconsole=1' before appending the id parameter at the back.