SharePoint 2007 MOSS WSS default.aspx HTTP 404 backup restore

Scenario / Symptoms:
A site collection is backed up and restored to a different farm. When users navigate to the site default.aspx they receive HTTP 404 error.
Users are able to visit the _layouts/settings.aspx page but not the default.aspx
Default.aspx on some subsites work and some do not.

Resolution:
Make absolutely sure that all custom solutions (wsp) are deployed at the farm level.

This took me a lot of time to figure this out because we never had documentation of all custom solutions on the farm.