When switching our website from HTTP to HTTPS, things went catastrophic:

* Page impressions went down from average 60 (peak 110) to below 10.
* Traffic from search engines dropped to zero.
* In other words, we became almost invisible on Google.

**What caveats may we have missed?**

This is what we did:

* Using a commercial SSL certificate (i.e. no self-signed or CAcert)
* Using the exact same domain name as before (i.e. no "www." before and after)
* Using 301 redirects for each HTTP page to its HTTPS version
* Internal links are all relative URLs (before and after)
* Same content, except for chaging "link rel=canonical" URLs from HTTP to HTTPS

As far as I can tell, we did everything that is recommended by SEO blogs and other StackExchange answers (such as http://webmasters.stackexchange.com/a/68089/48957).