Since Google gives a tiny ranking boost for HTTPS sites, many sites have begun switching over.

But if done incorrectly, it can cause a lot of confusion for your web site listings and rankings in Google.

One webmaster said Google switched over his site pretty quickly from HTTP to HTTPS but then shortly after, began indexing the HTTP version again. He said this in a Google Webmaster Help thread.

John Mueller, on New Years Day (first Googler to post in the help forums I believe), said this is because the canonical is set on some pages to go from HTTPS to HTTP. That is confusing Google.

You have a 301 redirect from HTTP to HTTPS and then you have a canonical tag, which is a 301 like signal, saying go from HTTPS to HTTP.

John wrote:

For more on this HTTPS stuff, see our HTTPS category.

Forum discussion at Google Webmaster Help.