I published a link to an article about security on Google and gmail earlier.

I added the 2-step verification process mentioned in the article to my Gmail account.   After this change, I was surprised that discover that Outlook and gmail on my iPad would not work.   A dialogue box kept telling me that my email or pw was incorrect. 

Revelation – As it turns out, you have to make a batch of changes to third party clients or your links to them will be blocked.  After about an hour, I figured it all out.   Google sends you an email about this, but it won’t reach your email client (Outlook for example) – you will have to go to gmail in to see it.
You can reverse the process, which I did, or you can add more to your existing PW.   Comment if you think that Google should tell you all about this before your make your changes!