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Disclaimer: I've only witnessed this behaviour on chrome. adding the rightmost dot to a URL as in should result in a fully qualified domain name (FQDN) and hence I wouldn't expect any problems when resolving that url. However chrome throws a security error the first time you're trying to access it.

is there an explanation for this error?

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up vote 2 down vote accepted

Resolving is not the problem.
Facebook uses by default HTTPS, the SSL certificate facebook uses at has a CN (Common Name) of not
Your browser checks the CN of the certificate against the hostname, it doesn't match and it throws a security warning.

I don't know why it's only the first time for you, I don't use Chrome, maybe it permanently saves your choice to accept this mismatch.
Or maybe you didn't notice that once you accepted it, facebook will actually redirect to

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